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How the Python import system works

How the Python import system works

From: https://tenthousandmeters.com/blog/python-behind-the-scenes-11-how-the-python-import-system-works/

If you ask me to name the most misunderstood aspect of Python, I will answer without a second thought: the Python import system. Just remember how many times you used relative imports and got something like ImportError: attempted relative import with no known parent package; or tried to figure out how to structure a project so that all the imports work correctly; or hacked sys.path when you couldn't find a better solution. Every Python programmer experienced something like this, and popular StackOverflow questions, such us Importing files from different folder (1822 votes), Relative imports in Python 3 (1064 votes) and Relative imports for the billionth time (993 votes), are a good indicator of that.

The Python import system doesn't just seem complicated – it is complicated. So even though the documentation is really good, it doesn't give you the full picture of what's going on. The only way to get such a picture is to study what happens behind the scenes when Python executes an import statement. And that's what we're going to do today.

Note: In this post I'm referring to CPython 3.9. Some implementation details will certainly change as CPython evolves. I'll try to keep track of important changes and add update notes.

Our plan

Before we begin, let me present you a more detailed version of our plan. First, we'll discuss the core concepts of the import system: modules, submodules, packages, from <> import <> statements, relative imports, and so on. Then we'll desugar different import statements and see that they all eventually call the built-in __import__() function. Finally, we'll study how the default implementation of __import__() works. Let's go!

Modules and module objects

Consider a simple import statement:

import m

What do you think it does? You may say that it imports a module named m and assigns the module to the variable m. And you'll be right. But what is a module exactly? What gets assigned to the variable? In order to answer these questions, we need to give a bit more precise explanation: the statement import m searches for a module named m, creates a module object for that module, and assigns the module object to the variable. See how we differentiated between a module and a module object. We can now define these terms.

module is anything that Python considers a module and knows how to create a module object for. This includes things like Python files, directories and built-in modules written in C. We'll look at the full list in the next section.

The reason why we import any module is because we want to get an access to functions, classes, constants and other names that the module defines. These names must be stored somewhere, and this is what module objects are for. A module object is a Python object that acts as a namespace for the module's names. The names are stored in the module object's dictionary (available as m.__dict__), so we can access them as attributes.

If you wonder how module objects are implemented, here's the definition from Objects/moduleobject.c:

typedef struct {
    PyObject ob_base;
    PyObject *md_dict;
    struct PyModuleDef *md_def;
    void *md_state;
    PyObject *md_weaklist;
    PyObject *md_name;
} PyModuleObject;

The md_dict field stores the module's dictionary. Other fields are not really important for our discussion.

Python creates module objects implicitly for us. To see that there is nothing magical about this process, let's create a module object ourselves. We usually create Python objects by calling their types, like MyClass() or set(). The type of a module object is PyModule_Type in the C code but it's not available in Python as a built-in. Fortunately, such "unavailable" types can be found in the types standard module:

$ python -q
>>> from types import ModuleType
>>> ModuleType
<class 'module'>

How does the types module define ModuleType? It just imports the sys module (any module will do) and then calls type() on the module object returned. We can do it as well:

>>> import sys
>>> ModuleType = type(sys)
>>> ModuleType
<class 'module'>

No matter how we get ModuleType, once we get it, we can easily create a module object:

>>> m = ModuleType('m')
>>> m
<module 'm'>

A newly created module object is not very interesting but has some special attributes preinitialized:

>>> m.__dict__
{'__name__': 'm', '__doc__': None, '__package__': None, '__loader__': None, '__spec__': None}

Most of these special attributes are mainly used by the import system itself, but some are used in the application code as well. The __name__ attribute, for example, is often used to get the name of the current module:

>>> __name__
'__main__'

Notice that __name__ is available as a global variable. This observation may seem evident, but it's crucial. It comes from the fact that the dictionary of global variables is set to the dictionary of the current module:

>>> import sys
>>> current_module = sys.modules[__name__] # sys.modules stores imported modules
>>> current_module.__dict__ is globals()
True

The current module acts as a namespace for the execution of Python code. When Python imports a Python file, it creates a new module object and then executes the contents of the file using the dictionary of the module object as the dictionary of global variables. Similarly, when Python executes a Python file directly, it first creates a special module called __main__ and then uses its dictionary as the dictionary of global variables. Thus, global variables are always attributes of some module, and this module is considered to be the current module from the perspective of the executing code.

Different kinds of modules

By default, Python recognizes the following things as modules:

  1. Built-in modules.
  2. Frozen modules.
  3. C extensions.
  4. Python source code files (.py files).
  5. Python bytecode files (.pyc files).
  6. Directories.

Built-in modules are C modules compiled into the python executable. Since they are part of the executable, they are always available. This is their key feature. The sys.builtin_module_names tuple stores their names:

$ python -q
>>> import sys
>>> sys.builtin_module_names
('_abc', '_ast', '_codecs', '_collections', '_functools', '_imp', '_io', '_locale', '_operator', '_peg_parser', '_signal', '_sre', '_stat', '_string', '_symtable', '_thread', '_tracemalloc', '_warnings', '_weakref', 'atexit', 'builtins', 'errno', 'faulthandler', 'gc', 'itertools', 'marshal', 'posix', 'pwd', 'sys', 'time', 'xxsubtype')

Frozen modules are too a part of the python executable, but they are written in Python. Python code is compiled to a code object and then the marshalled code object is incorporated into the executable. The examples of frozen modules are _frozen_importlib and _frozen_importlib_external . Python freezes them because they implement the core of the import system and, thus, cannot be imported like other Python files.

C extensions are a bit like built-in modules and a bit like Python files. On one hand, they are written in C or C++ and interact with Python via the Python/C API. On the other hand, they are not a part of the executable but loaded dynamically during the import. Some standard modules including arraymath and select are C extensions. Many others including asyncioheapq and json are written in Python but call C extensions under the hood. Technically, C extensions are shared libraries that expose a so called initialization function. They are usually named like modname.so, but the file extension may be different depending on the platform. On my macOS, for example, any of these extensions will work: .cpython-39-darwin.so.abi3.so.so. And on Windows, you'll see .dll and its variations.

Python bytecode files are typically live in a __pycache__ directory alongside regular Python files. They are the result of compiling Python code to bytecode. More specifically, a .pyc file contains some metadata followed by a marshalled code object of a module. Its purpose is to reduce module's loading time by skipping the compilation stage. When Python imports a .py file, it first searches for a corresponding .pyc file in the __pycache__ directory and executes it. If the .pyc file does not exist, Python compiles the code and creates the file.

However, we wouldn't call .pyc files modules if we couldn't execute and import them directly. Surprisingly, we can:

$ ls
module.pyc
$ python module.pyc 
I'm a .pyc file
$ python -c "import module"
I'm a .pyc file

To learn more about .pyc files, check out PEP 3147 -- PYC Repository Directories and PEP 552 -- Deterministic pycs.

As we'll later see, we can customize the import system to support even more kinds of modules. So anything can be called a module as long as Python can create a module object for it given a module name.

Submodules and packages

If module names were limited to simple identifiers like mymodule or utils, then they all must have been unique, and we would have to think very hard every time we give a new file a name. For this reason, Python allows modules to have submodules and module names to contain dots.

When Python executes this statements:

import a.b

it first imports the module a and then the submodule a.b. It adds the submodule to the module's dictionary and assigns the module to the variable a, so we can access the submodule as a module's attribute.

A module that can have submodules is called a package. Technically, a package is a module that has a __path__ attribute. This attribute tells Python where to look for submodules. When Python imports a top-level module, it searches for the module in the directories and ZIP archives listed in sys.path. But when it imports a submodule, it uses the __path__ attribute of the parent module instead of sys.path.

Regular packages

Directories are the most common way to organize modules into packages. If a directory contains a __init__.py file, it's considered to be a regular package. When Python imports such a directory, it executes the __init__.py file, so the names defined there become the attributes of the module.

The __init__.py file is typically left empty or contains package-related attributes such as __doc__ and __version__. It can also be used to decouple the public API of a package from its internal implementation. Suppose you develop a library with the following structure:

mylibrary/
    __init__.py
    module1.py
    module2.py

And you want to provide the users of your library with two functions: func1() defined in module1.py and func2() defined in module2.py. If you leave __init__.py empty, then the users must specify the submodules to import the functions:

from mylibrary.module1 import func1
from mylibrary.module2 import func2

It may be something you want, but you may also want to allow the users to import the functions like this:

from mylibrary import func1, func2

So you import the functions in __init__.py:

# mylibrary/__init__.py
from mylibrary.module1 import func1
from mylibrary.module2 import func2

A directory with a C extension named __init__.so or with a .pyc file named __init__.pyc is also a regular package. Python can import such packages perfectly fine:

$ ls
spam
$ ls spam/
__init__.so
$ python -q
>>> import spam
>>> 

Namespace packages

Before version 3.3, Python had only regular packages. Directories without __init__.py were not considered packages at all. And this was a problem because people didn't like to create empty __init__.py files. PEP 420 made these files unnecessary by introducing namespace packages in Python 3.3.

Namespace packages solved another problem as well. They allowed developers to place contents of a package across multiple locations. For example, if you have the following directory structure:

mylibs/
    company_name/
        package1/...
morelibs/
    company_name/
        package2/...

And both mylibs and morelibs are in sys.path, then you can import both package1 and package2 like this:

>>> import company_name.package1
>>> import company_name.package2

This is because company_name is a namespace package that contains two locations:

>>> company_name.__path__
_NamespacePath(['/morelibs/company_name', '/mylibs/company_name'])

How does it work? When Python traverses path entries in the path (sys.path or parent's __path__) during the module search, it remembers the directories without __init__.py that match the module's name. If after traversing all the entries, it couldn't find a regular package, a Python file or a C extension, it creates a module object whose __path__ contains the memorized directories.

The initial idea of requiring __init__.py was to prevent directories named like string or site from shadowing standard modules. Namespace package do not shadow other modules because they have lower precedence during the module search.

Importing from modules

Besides importing modules, we can also import module attributes using a from <> import <> statement, like so:

from module import func, Class, submodule

This statement imports a module named module and assign the specified attributes to the corresponding variables:

func = module.func
Class = module.Class
submodule = module.submodule

Note that the module variable is not available after the import as if it was deleted:

del module

When Python sees that a module doesn't have a specified attribute, it considers the attribute to be a submodule and tries to import it. So if module defines func and Class but not submodule, Python will try to import module.submodule.

Wildcard imports

If we don't want to specify explicitly the names to import from a module, we can use the wildcard form of import:

from module import *

This statement works as if "*" was replaced with all the module's public names. These are the names in the module's dictionary that do not start with an underscore "_" or the names listed in the __all__ attribute if it's defined.

Relative imports

Up until now we've been telling Python what modules to import by specifying absolute module names. The from <> import <> statement allows us to specify relative module names as well. Here are a few examples:

from . import a
from .. import a
from .a import b
from ..a.b import c

The constructions like .. and ..a.b are relative module names, but what are they relative to? As we said, a Python file is executed in the context of the current module whose dictionary acts as a dictionary of global variables. The current module, as any other module, can belong to a package. This package is called the current package, and this is what relative module names are relative to.

The __package__ attribute of a module stores the name of the package to which the module belongs. If the module is a package, then the module belongs to itself, and __package__ is just the module's name (__name__). If the module is a submodule, then it belongs to the parent module, and __package__ is set to the parent module's name. Finally, if the module is not a package nor a submodule, then its package is undefined. In this case, __package__ can be set to an empty string (e.g. the module is a top-level module) or None (e.g. the module runs as a script).

A relative module name is a module name preceded by some number of dots. One leading dot represents the current package. So, when __package__ is defined, the following statement:

from . import a

works as if the dot was replaced with the value of __package__.

Each extra dot tells Python to move one level up from __package__ . If __package__ is set to "a.b", then this statement:

from .. import d

works as if the dots were replaced with a.

You cannot move outside the top-level package. If you try this:

from ... import e

Python will throw an error:

ImportError: attempted relative import beyond top-level package

This is because Python does not move through the file system to resolve relative imports. It just takes the value of __package__, strips some suffix and appends a new one to get an absolute module name.

Obviously, relative imports break when __package__ is not defined at all. In this case, you get the following error:

ImportError: attempted relative import with no known parent package

You most commonly see it when you run a program with relative imports as a script. Since you specify which program to run with a filesystem path and not with a module name, and since Python needs a module name to calculate __package__, the code is executed in the __main__ module whose __package__ attribute is set to None.

Running programs as modules

The standard way to avoid import errors when running a program with relative imports is to run it as a module using the -m switch:

$ python -m package.module

The -m switch tells Python to use the same mechanism to find the module as during the import. Python gets a module name and is able to calculate the current package. For example, if we run a module named package.module, where module refers to a regular .py file, then the code will be executed in the __main__ module whose __package__ attribute is set to "package". You can read more about the -m switch in the docs and in PEP 338.

Alright. This was a warm-up. Now we're going to see what exactly happens when we import a module.

Desugaring the import statement

If we desugar any import statement, we'll see that it eventually calls the built-in __import__() function. This function takes a module name and a bunch of other parameters, finds the module and returns a module object for it. At least, this is what it's supposed to do.

Python allows us to set __import__() to a custom function, so we can change the import process completely. Here's, for example, a change that just breaks everything:

>>> import builtins
>>> builtins.__import__ = None
>>> import math
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'NoneType' object is not callable

You rarely see people overriding __import__() for reasons other than logging or debugging. The default implementation already provides powerful mechanisms for customization, and we'll focus solely on it.

The default implementation of __import__() is importlib.__import__(). Well, it's almost true. The importlib module is a standard module that implements the core of the import system. It's written in Python because the import process involves path handling and other things that you would prefer to do in Python rather than in C. But some functions of importlib are ported to C for performance reasons. And default __import__() actually calls a C port of importlib.__import__(). For our purposes, we can safely ingore the difference and just study the Python version. Before we do that, let's see how different import statements call __import__().

Simple imports

Recall that a piece of Python code is executed in two steps:

  1. The compiler compiles the code to bytecode.
  2. The VM executes the bytecode.

To see what an import statement does, we can look at the bytecode produced for it and then find out what each bytecode instruction does by looking at the evaluation loop in Python/ceval.c.

To get the bytecode, we use the dis standard module:

$ echo "import m" | python -m dis
  1           0 LOAD_CONST               0 (0)
              2 LOAD_CONST               1 (None)
              4 IMPORT_NAME              0 (m)
              6 STORE_NAME               0 (m)
...

The first LOAD_CONST instruction pushes 0 onto the value stack. The second LOAD_CONST pushes None. Then the IMPORT_NAME instruction does something we'll look into in a moment. Finally, STORE_NAME assigns the value on top of the value stack to the variable m.

The code that executes the IMPORT_NAME instruction looks as follows:

case TARGET(IMPORT_NAME): {
    PyObject *name = GETITEM(names, oparg);
    PyObject *fromlist = POP();
    PyObject *level = TOP();
    PyObject *res;
    res = import_name(tstate, f, name, fromlist, level);
    Py_DECREF(level);
    Py_DECREF(fromlist);
    SET_TOP(res);
    if (res == NULL)
        goto error;
    DISPATCH();
}

All the action happens in the import_name() function. It calls __import__() to do the work, but if __import__() wasn't overridden, it takes a shortcut and calls the C port of importlib.__import__() called PyImport_ImportModuleLevelObject(). Here's how this logic is implemented in the code:

static PyObject *
import_name(PyThreadState *tstate, PyFrameObject *f,
            PyObject *name, PyObject *fromlist, PyObject *level)
{
    _Py_IDENTIFIER(__import__);
    PyObject *import_func, *res;
    PyObject* stack[5];

    import_func = _PyDict_GetItemIdWithError(f->f_builtins, &PyId___import__);
    if (import_func == NULL) {
        if (!_PyErr_Occurred(tstate)) {
            _PyErr_SetString(tstate, PyExc_ImportError, "__import__ not found");
        }
        return NULL;
    }

    /* Fast path for not overloaded __import__. */
    if (import_func == tstate->interp->import_func) {
        int ilevel = _PyLong_AsInt(level);
        if (ilevel == -1 && _PyErr_Occurred(tstate)) {
            return NULL;
        }
        res = PyImport_ImportModuleLevelObject(
                        name,
                        f->f_globals,
                        f->f_locals == NULL ? Py_None : f->f_locals,
                        fromlist,
                        ilevel);
        return res;
    }

    Py_INCREF(import_func);

    stack[0] = name;
    stack[1] = f->f_globals;
    stack[2] = f->f_locals == NULL ? Py_None : f->f_locals;
    stack[3] = fromlist;
    stack[4] = level;
    res = _PyObject_FastCall(import_func, stack, 5);
    Py_DECREF(import_func);
    return res;
}

If you carefully examine all of the above, you'll be able to conclude that this statement:

import m

is actually equivalent to this code:

m = __import__('m', globals(), locals(), None, 0)

the meaning of the arguments according to the docstring of importlib.__import__() being the following:

def __import__(name, globals=None, locals=None, fromlist=(), level=0):
    """Import a module.

    The 'globals' argument is used to infer where the import is occurring from
    to handle relative imports. The 'locals' argument is ignored. The
    'fromlist' argument specifies what should exist as attributes on the module
    being imported (e.g. ``from module import <fromlist>``).  The 'level'
    argument represents the package location to import from in a relative
    import (e.g. ``from ..pkg import mod`` would have a 'level' of 2).

    """

As we said, all import statements eventually call __import__(). They differ in what they do before and after the call and how they make the call. Relative imports, for example, pass non-zero level, and from <> import <> statements pass non-empty fromlist.

Let's now express other import statements via __import__() as we expressed import m but much faster this time.

Importing submodules

This statement:

import a.b.c

compiles to the following bytecode:

$ echo "import a.b.c" | python -m dis  
  1           0 LOAD_CONST               0 (0)
              2 LOAD_CONST               1 (None)
              4 IMPORT_NAME              0 (a.b.c)
              6 STORE_NAME               1 (a)
...

and is equivalent to the following code:

a = __import__('a.b.c', globals(), locals(), None, 0)

The arguments to __import__() are passed in the same way as in the case of import m. The only difference is that the VM assigns the result of __import__() not to the name of the module (a.b.c is not a valid variable name) but to the first identifier before the dot, i.e. a. As we'll see, __import__() returns the top-level module in this case.

from <> import <>

This statement:

from a.b import f, g

compiles to the following bytecode:

$ echo "from a.b import f, g" | python -m dis  
  1           0 LOAD_CONST               0 (0)
              2 LOAD_CONST               1 (('f', 'g'))
              4 IMPORT_NAME              0 (a.b)
              6 IMPORT_FROM              1 (f)
              8 STORE_NAME               1 (f)
             10 IMPORT_FROM              2 (g)
             12 STORE_NAME               2 (g)
             14 POP_TOP
...

and is equivalent to the following code:

a_b = __import__('a.b', globals(), locals(), ('f', 'g'), 0)
f = a_b.f
g = a_b.g
del a_b

The names to import are passed as fromlist. When fromlist is not empty, __import__() returns not the top-level module as in the case of a simple import but the specified module like a.b.

from <> import *

This statement:

from m import *

compiles to the following bytecode:

$ echo "from m import *" | python -m dis
  1           0 LOAD_CONST               0 (0)
              2 LOAD_CONST               1 (('*',))
              4 IMPORT_NAME              0 (m)
              6 IMPORT_STAR
...

and is equivalent to the following code:

m = __import__('m', globals(), locals(), ('*',), 0)
all_ = m.__dict__.get('__all__')
if all_ is None:
    all_ = [k for k in m.__dict__.keys() if not k.startswith('_')]
for name in all_:
    globals()[name] = getattr(m, name)
del m, all_, name

The __all__ attribute lists all public names of the module. If some names listed in __all__ are not defined, __import__() tries to import them as submodules.

Relative imports

This statement:

from .. import f

compiles to the following bytecode

$ echo "from .. import f" | python -m dis
  1           0 LOAD_CONST               0 (2)
              2 LOAD_CONST               1 (('f',))
              4 IMPORT_NAME              0
              6 IMPORT_FROM              1 (f)
              8 STORE_NAME               1 (f)
             10 POP_TOP
...

and is equivalent to the following code:

m = __import__('', globals(), locals(), ('f',), 2)
f = m.f
del m

The level argument tells __import__() how many leading dots the relative import has. Since it's set to 2__import__() calculates the absolute name of the module by (1) taking the value of __package__ and (2) stripping its last portion. The __package__ attribute is available to __import__() because it's passed with globals().

We're now done with import statements and can focus solely on the __import__() function.

Inside __import__()

As I learned preparing this article, studying __import__() by following all its code paths is not the most entertaining experience. So I offer you a better option. I'll summarize the key algorithms of the import process in plain English and give links to the functions that implement these algorithms so you can read the code if something is left unclear.

The algorithm that __import__() implements can be summarized as follows:

  1. If level > 0, resolve a relative module name to an absolute module name.
  2. Import the module.
  3. If fromlist is empty, drop everything after the first dot from the module name to get the name of the top-level module. Import and return the top-level module.
  4. If fromlist contains names that are not in the module's dictionary, import them as submodules. That is, if submodule is not in the module's dictionary, import module.submodule. If "*" is in fromlist, use module's __all__ as new fromlist and repeat this step.
  5. Return the module.

Step 2 is where all the action happens. We'll focus on it in the remaining sections, but let us first elaborate on step 1.

Resolving relative names

To resolve a relative module name, __import__() needs to know the current package of the module from which the import statement was executed. So it looks up __package__ in globals. If __package__ is None__import__() tries to deduce the current package from __name__. Since Python always sets __package__ correctly, this fallback is typically unnecessary. It can only be useful for modules created by means other than the default import mechanism. You can look at the _calc___package__() function to see how the current package is calculated exactly. All we should remember is that relative imports break when __package__ is set to an empty string, as in the case of a top-level module, or to None, as in the case of a script, and have a chance of succeeding otherwise. The following function ensures this:

def _sanity_check(name, package, level):
    """Verify arguments are "sane"."""
    if not isinstance(name, str):
        raise TypeError('module name must be str, not {}'.format(type(name)))
    if level < 0:
        raise ValueError('level must be >= 0')
    if level > 0:
        if not isinstance(package, str):
            raise TypeError('__package__ not set to a string')
        elif not package:
            raise ImportError('attempted relative import with no known parent '
                              'package')
    if not name and level == 0:
        raise ValueError('Empty module name')

After the check, the relative name gets resolved:

def _resolve_name(name, package, level):
    """Resolve a relative module name to an absolute one."""
    # strip last `level - 1` portions of `package`
    bits = package.rsplit('.', level - 1)
    if len(bits) < level:
        # stripped less than `level - 1` portions
        raise ImportError('attempted relative import beyond top-level package')
    base = bits[0]
    return '{}.{}'.format(base, name) if name else base

And __import__() calls _find_and_load() to import the module.

The import process

The _find_and_load() function takes an absolute module name and performs the following steps:

  1. If the module is in sys.modules, return it.
  2. Initialize the module search path to None.
  3. If the module has a parent module (the name contains at least one dot), import the parent module if it's not in sys.modules yet. Set the module search path to parent's __path__.
  4. Find the module's spec using the module name and the module search path. If the spec is not found, raise ModuleNotFoundError.
  5. Load the module from the spec.
  6. Add the module to the dictionary of the parent module.
  7. Return the module.

All imported modules are stored in the sys.modules dictionary. This dictionary maps module names to module objects and acts as a cache. Before searching for a module, _find_and_load() checks sys.modules and returns the module immideatly if it's there. Imported modules are added to sys.module at the end of step 5.

If the module is not in sys.modules_find_and_load() proceeds with the import process. This process consists of finding the module and loading the module. Finders and loaders are objects that perform these tasks.

Finders and loaders

The job of a finder is to make sure that the module exists, determine which loader should be used for loading the module and provide the information needed for loading, such as a module's location. The job of a loader is to create a module object for the module and execute the module. The same object can function both as a finder and as a loader. Such an object is called an importer.

Finders implement the find_spec() method that takes a module name and a module search path and returns a module spec. A module spec is an object that encapsulates the loader and all the information needed for loading. This includes module's special attributes. They are simply copied from the spec after the module object is created. For example, __path__ is copied from spec.submodule_search_locations, and __package__ is copied from spec.parent. See the docs for the full list of spec attributes.

To find a spec, _find_and_load() iterates over the finders listed in sys.meta_path and calls find_spec() on each one until the spec is found. If the spec is not found, _find_and_load() raises ModuleNotFoundError.

By default, sys.meta_path stores three finders:

  1. BuiltinImporter that searches for built-in modules
  2. FrozenImporter that searches for frozen modules; and
  3. PathFinder that searches for different kinds of modules including Python files, directories and C extensions.

These are called meta path finders. Python differentiates them from path entry finders that are a part of PathFinder. We'll discuss both types of finders in the next sections.

After the spec is found, _find_and_load() takes the loader from the spec and passes the spec to the loader's create_module() method to create a module object. If create_module() is not implemented or returns None, then _find_and_load() creates the new module object itself. If the module object does not define some special attributes, which is usually the case, the attributes are copied from the spec. Here's how this logic is implemented in the code:

def module_from_spec(spec):
    """Create a module based on the provided spec."""
    # Typically loaders will not implement create_module().
    module = None
    if hasattr(spec.loader, 'create_module'):
        # If create_module() returns `None` then it means default
        # module creation should be used.
        module = spec.loader.create_module(spec)
    elif hasattr(spec.loader, 'exec_module'):
        raise ImportError('loaders that define exec_module() '
                          'must also define create_module()')
    if module is None:
        # _new_module(name) returns type(sys)(name)
        module = _new_module(spec.name)

    # copy undefined module attributes (__loader__, __package__, etc.)
    # from the spec
    _init_module_attrs(spec, module)
    return module

After creating the module object, _find_and_load() executes the module by calling the loader's exec_module() method. What this method does depends on the loader, but typically it populates the module's dictionary with functions, classes, constants and other things that the module defines. The loader of Python files, for example, executes the contents of the file when exec_module() is called.

The full loading process is implemented as follows:

def _load_unlocked(spec):
    # ... compatibility stuff

    module = module_from_spec(spec)

    # needed for parallel imports
    spec._initializing = True
    try:
        sys.modules[spec.name] = module
        try:
            if spec.loader is None:
                if spec.submodule_search_locations is None:
                    raise ImportError('missing loader', name=spec.name)
                # A namespace package so do nothing.
            else:
                spec.loader.exec_module(module)
        except:
            try:
                del sys.modules[spec.name]
            except KeyError:
                pass
            raise
        # Move the module to the end of sys.modules.
        # This is to maintain the import order.
        # Yeah, Python dicts are ordered
        module = sys.modules.pop(spec.name)
        sys.modules[spec.name] = module
        _verbose_message('import {!r} # {!r}', spec.name, spec.loader)
    finally:
        spec._initializing = False

    return module

This piece of code is interesting for several reasons. First, a module is added to sys.modules before it is executed. Due to this logic, Python supports circular imports. If we have two modules that import each other like this:

# a.py
import b

X = "some constant"
# b.py
import a

We can import them without any issues:

$ python -q
>>> import a
>>> 

The catch is that the module a is only partially initialized when the module b is executed. So if we use a.X in b:

# b.py
import a

print(a.X)

we get an error:

$ python -q
>>> import a
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/a.py", line 1, in <module>
    import b
  File "/b.py", line 3, in <module>
    print(a.X)
AttributeError: partially initialized module 'a' has no attribute 'X' (most likely due to a circular import)

Second, a module is removed from sys.modules if the execution fails for any reason, but modules that were successfully imported as a side-effect remain in sys.modules.

Finally, the module in sys.modules can be replaced during the module execution. Thus, the module is looked up in sys.modules before it's returned.

We're now done with _find_and_load() and __import__() and ready to see how different finders and loaders work.

BuiltinImporter and FrozenImporter

As we can judge from the name, BuiltinImporter is both a finder and a loader of built-in modules. Its find_spec() method checks if the module is a built-in module and if so, creates a spec that contains nothing but the module's name and the loader. Its create_module() method finds the module's init function and calls it. Both methods are easy to implement because built-in module names are statically mapped to init functions:

struct _inittab _PyImport_Inittab[] = {
    {"posix", PyInit_posix},
    {"errno", PyInit_errno},
    {"pwd", PyInit_pwd},
    {"_sre", PyInit__sre},
    {"_codecs", PyInit__codecs},
    {"_weakref", PyInit__weakref},
    {"_functools", PyInit__functools},
    {"_operator", PyInit__operator},
    {"_collections", PyInit__collections},
    {"_abc", PyInit__abc},
    {"itertools", PyInit_itertools},
    {"atexit", PyInit_atexit},
    // ... more entries
};

The init functions are the same init functions that C extensions define. We're not going to discuss how they work here, so if you want to learn more about this, check out the Extending Python with C or C++ tutorial.

FrozenImporter finds frozen modules in the same way. Their names are statically mapped to code objects:

static const struct _frozen _PyImport_FrozenModules[] = {
    /* importlib */
    {"_frozen_importlib", _Py_M__importlib_bootstrap,
        (int)sizeof(_Py_M__importlib_bootstrap)},
    {"_frozen_importlib_external", _Py_M__importlib_bootstrap_external,
        (int)sizeof(_Py_M__importlib_bootstrap_external)},
    {"zipimport", _Py_M__zipimport,
        (int)sizeof(_Py_M__zipimport)},
    /* Test module */
    {"__hello__", M___hello__, SIZE},
    /* Test package (negative size indicates package-ness) */
    {"__phello__", M___hello__, -SIZE},
    {"__phello__.spam", M___hello__, SIZE},
    {0, 0, 0} /* sentinel */
};

The difference with BuiltinImporter is that create_module() returns None. Code objects are executed by exec_module().

We now focus on the meta path finder that application developers should care about the most.

PathFinder

PathFinder searches for modules on the module search path. The module search path is parent's __path__ passed as the path argument to find_spec() or sys.path if this argument is None. It's expected to be an iterable of strings. Each string, called a path entry, should specify a location to search for modules, such as a directory on the file system.

PathFinder doesn't actually do the search itself but associates each path entry with a path entry finder that knows how to find modules in the location specified by the path entry. To find a module, PathFinder iterates over the path entries and, for each entry, calls find_spec() of the corresponding path entry finder.

To find out which path entry finder to use for a particular entry, PathFinder calls path hooks listed in sys.path_hooks. A path hook is a callable that takes a path entry and returns a path entry finder. It can also raise ImportError, in which case PathFinder tries the next hook. To avoid calling hooks on each import, PathFinder caches the results in the sys.path_importer_cache dictionary that maps path entries to path entry finders.

By default, sys.path_hooks contains two path hooks:

  1. a hook that returns zipimporter instances; and
  2. a hook that returns FileFinder instances.

zipimporter instance searches for modules in a ZIP archive or in a directory inside a ZIP archive. It supports the same kinds of modules as FileFinder except for C extensions. You can read more about zipimporter in the docs and in PEP 273. A FileFinder instance searches for modules in a directory. We'll discuss it in the next section.

Besides calling path entry finders, PathFinder creates specs for namespace packages. When a path entry finder returns a spec that doesn't specify a loader, this means that the spec describes a portion of a namespace package (typically just a directory). In this case, PathFinder remembers the submodule_search_locations attribute of this spec and continues with the next path entry hoping that it will find a Python file, a regular package or a C extension. If it doesn't find any of these eventually, it creates a new spec for a namespace package whose submodule_search_locations contains all the memorized portions.

To sum up what we said about PathFinder, here's the complete algorithm that its find_spec() implements:

  1. If path is None, set path to sys.path.
  2. Initialize the list of path entries of a potential namespace package: namespace_path = [].
  3. For each path entry in path:
    1. Look up the entry in sys.path_importer_cache to get a path entry finder.
    2. If the entry is not in sys.path_importer_cache, call hooks listed in sys.path_hooks until some hook returns a path entry finder.
    3. Store the path entry finder in sys.path_importer_cache. If no path entry finder is found, store None and continue with the next entry.
    4. Call find_spec() of the path entry finder. If the spec is None, continue with the next entry.
    5. If found a namespace package (spec.loader is None), extend namespace_path with spec.submodule_search_locations and continue with the next entry.
    6. Otherwise, return the spec.
  4. If namespace_path is empty, return None.
  5. Create a new namespace package spec with submodule_search_locations based on namespace_path.
  6. Return the spec.

All this complicated logic of PathFinder is unnecessary most of the time. Typically, a path entry is just a path to a directory, so PathFinder calls the find_spec() method of a FileFinder instance returned by the corresponding hook.

FileFinder

FileFinder instance searches for modules in the directory specified by the path entry. A path entry can either be an absolute path or a relative path. In the latter case, it's resolved with respect to the current working directory.

The find_spec() method of FileFinder takes an absolute module name but needs only the "tail" portion after the last dot since the package portion was already used to determine the directory to search in. It extracts the "tail" like this:

modname_tail = modname.rpartition('.')[2]

Then it performs the search. It looks for a directory named {modname_tail} that contains __init__.py__init__.pyc or __init__ with some shared library file extension like .so. It also looks for files named {modname_tail}.py{modname_tail}.pyc and {modname_tail}.{any_shared_library_extension}. If it finds any of these, it creates a spec with the corresponding loader:

  • ExtensionFileLoader for a C extension
  • SourceFileLoader for a .py file; and
  • SourcelessFileLoader for a .pyc file.

If it finds a directory that is not a regular package, it creates a spec with the loader set to NonePathFinder collects a single namespace package spec from such specs.

The algorithm that find_spec() implements can be summarized as follows:

  1. Get the last portion of the module name: modname_tail = modname.rpartition('.')[2].
  2. Look for a directory named {modname_tail} that contains __init__.{any_shared_library_extension}. If found, create and return a regular package spec.
  3. Look for a file named {modname_tail}.{any_shared_library_extension} If found, create and return a file spec.
  4. Repeat steps 2 and 3 for .py files and for .pyc files.
  5. If found a directory named {modname_tail} that is not a regular package, create and return a namespace package spec.
  6. Otherwise, return None.

A regular package spec is created like this:

loader = SourceFileLoader(modname, path_to_init) # loader may be different
spec = ModuleSpec(modname, loader, origin=path_to_init)
spec.submodule_search_locations = [path_to_package]

a file spec like this:

loader = SourceFileLoader(modname, path_to_file) # loader may be different
spec = ModuleSpec(modname, loader, origin=path_to_file)
spec.submodule_search_locations = None

and a namespace package like this:

spec = ModuleSpec(modname, loader=None, origin=None)
spec.submodule_search_locations = [path_to_package]

Once the spec is created, the loading of the module begins. ExtensionFileLoader is worth studying, but we should leave it for another post on C extensions. SourcelessFileLoader is not very interesting, so we won't discuss it either. SourceFileLoader is the most relevant for us because it loads .py files. We'll briefly mention how it works.

SourceFileLoader

The create_module() method of SourceFileLoader always returns None. This means that _find_and_load() creates the new module object itself and initializes it by copying the attributes from the spec.

The exec_module() method of SourceFileLoader does exactly what you would expect:

def exec_module(self, module):
    """Execute the module."""
    code = self.get_code(module.__name__)
    if code is None:
        raise ImportError('cannot load module {!r} when get_code() '
                        'returns None'.format(module.__name__))
    _bootstrap._call_with_frames_removed(exec, code, module.__dict__)

It calls get_code() to create a code object from the file and then calls exec() to execute the code object in the module's namespace. Note that get_code() first tries to read the bytecode from the .pyc file in the __pycache__ directory and creates this file if it doesn't exist yet.

That's it! We completed our study of finders and loaders and saw what happens during the import process. Let's summarize what we've learned.

Summary of the import process

Any import statement compiles to a series of bytecode instructions, one of which, called IMPORT_NAME, imports the module by calling the built-in __import__() function. If the module was specified with a relative name, __import__() first resolves the relative name to an absolute one using the __package__ attribute of the current module. Then it looks up the module in sys.modules and returns the module if it's there. If the module is not there, __import__() tries to find the module's spec. It calls the find_spec() method of every finder listed in sys.meta_path until some finder returns the spec. If the module is a built-in module, BuiltinImporter returns the spec. If the module is a frozen module, FrozenImporter returns the spec. Otherwise, PathFinder searches for the module on the module search path, which is either the __path__ attribute of the parent module or sys.path if the former is undefined. PathFinder iterates over the path entries and, for each entry, calls the find_spec() method of the corresponding path entry finder. To get the corresponding path entry finder, PathFinder passes the path entry to callables listed in sys.path_hooks. If the path entry is a path to a directory, one of the callables returns a FileFinder instance that searches for modules in that directory. PathFinder calls its find_spec(). The find_spec() method of FileFinder checks if the directory specified by the path entry contains a C extension, a .py file, a .pyc file or a directory whose name matches the module name. If it finds anything, it create a module spec with the corresponding loader. When __import__() gets the spec, it calls the loader's create_module() method to create a module object and then the exec_module() method to execute the module. Finally, it puts the module in sys.modules and returns the module.

Do you have any questions left? I have one.

What's in sys.path?

By default, sys.path includes the following:

  1. An invocation-dependent current directory. If you run a program as a script, it's the directory where the script is located. If you run a program as a module using the -m switch, it's the directory from which you run the python executable. If you run python in the interactive mode or execute a command using the -c switch, the first entry in sys.path will be an empty string.
  2. Directories specified by the PYTHONPATH environment variable.
  3. A zip archive that contains the standard library, e.g. /usr/local/lib/python39.zip. It's used for embeddable installations. Normal installation do not include this archive.
  4. A directory that contains standard modules written in Python, e.g. /usr/local/lib/python3.9.
  5. A directory that contains standard C extensions, e.g. /usr/local/lib/python3.9/lib-dynload.
  6. Site-specific directories added by the site module, e.g. /usr/local/lib/python3.9/site-packages. That's where third-party modules installed by tools like pip go.

To construct these paths, Python first determines the location of the python executable. If we run the executable by specifying a path, Python already knows the location. Otherwise, it searches for the executable in PATH. Eventually, it gets something like /usr/local/bin/python3. Then it tries to find out where the standard modules are located. It moves one directory up from the executable until it finds the lib/python{X.Y}/os.py file. This file denotes the directory containing standard modules written in Python. The same process is repeated to find the directory containing standard C extensions, but the lib/python{X.Y}/lib-dynload/ directory is used as a marker this time. A pyvenv.cfg file alongside the executable or one directory up may specify another directory to start the search from. And the PYTHONHOME environment variable can be used to specify the "base" directory so that Python doesn't need to perform the search at all.

The site standard module takes the "base" directory found during the search or specified by PYTHONHOME and prepends lib/python{X.Y}/site-packages to it to get the directory containing third-party modules. This directory may contain .pth path configuration files that tell site to add more site-specific directories to sys.path. The added directories may contain .pth files as well so that the process repeats recursively.

If the pyvenv.cfg file exists, site uses the directory containing this file as the "base" directory. Note that this is not the directory that pyvenv.cfg specifies. By this mechanism, Python supports virtual environments that have their own site-specific directories but share the standard library with the system-wide installation. Check out the docs on site and PEP 405 -- Python Virtual Environments to learn more about this.

The process of calculating sys.path is actually even more nuanced. If you want to know those nuances, see this StackOverflow answer.

Conclusion

If you ask me to name the most misunderstood aspect of Python, I will answer without a second thought: the Python import system. Until I wrote this post, I couldn't really tell what a module is exactly; what a package is; what relative imports are relative to; how various customization points such as sys.meta_pathsys.path_hooks and sys.path fit together; and how sys.path is calculated. What can I tell now? First, modules and packages are simple concepts. I blame my misunderstanding on the docs that oversimplify the reality like this:

A module is a file containing Python definitions and statements.

or omit the details like this:

You can think of packages as the directories on a file system and modules as files within directories, but don’t take this analogy too literally since packages and modules need not originate from the file system. For the purposes of this documentation, we’ll use this convenient analogy of directories and files.

Relative imports are indeed unintuitive, but once you understand that they are just a way to specify a module name relative to the current package name, you should have no problems with them.

Meta path finders, path entry finders, path hooks, path entries and loaders make the import system more complex but also make it more flexible. PEP 302 and PEP 451 give some rationale for this trade-off.

What's about sys.path? It's crucial to understand what's there when you import a module, yet I couldn't find a satisfactory explanation in the docs. Perhaps, it's too complicated to describe precisely. But I think that the approximation like the one we gave in the previous section is good enough for practical purposes.

Overall, studying the import system was useful, but I think that the next time we should study something more exciting. How about async/await?


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