5. The import system¶
Python code in one module gains access to the code in another module
by the process of importing it. The import
statement is
the most common way of invoking the import machinery, but it is not the only
way. Functions such as importlib.import_module()
and built-in
__import__()
can also be used to invoke the import machinery.
The import
statement combines two operations; it searches for the
named module, then it binds the results of that search to a name in the local
scope. The search operation of the import
statement is defined as
a call to the __import__()
function, with the appropriate arguments.
The return value of __import__()
is used to perform the name
binding operation of the import
statement. See the
import
statement for the exact details of that name binding
operation.
A direct call to __import__()
performs only the module search and, if
found, the module creation operation. While certain side-effects may occur,
such as the importing of parent packages, and the updating of various caches
(including sys.modules
), only the import
statement performs
a name binding operation.
When calling __import__()
as part of an import statement, the
import system first checks the module global namespace for a function by
that name. If it is not found, then the standard builtin __import__()
is called. Other mechanisms for invoking the import system (such as
importlib.import_module()
) do not perform this check and will always
use the standard import system.
When a module is first imported, Python searches for the module and if found,
it creates a module object [1], initializing it. If the named module
cannot be found, an ImportError
is raised. Python implements various
strategies to search for the named module when the import machinery is
invoked. These strategies can be modified and extended by using various hooks
described in the sections below.
Changed in version 3.3: The import system has been updated to fully implement the second phase
of PEP 302. There is no longer any implicit import machinery - the full
import system is exposed through sys.meta_path
. In addition,
native namespace package support has been implemented (see PEP 420).
5.1. importlib
¶
The importlib
module provides a rich API for interacting with the
import system. For example importlib.import_module()
provides a
recommended, simpler API than built-in __import__()
for invoking the
import machinery. Refer to the importlib
library documentation for
additional detail.
5.2. Packages¶
Python has only one type of module object, and all modules are of this type, regardless of whether the module is implemented in Python, C, or something else. To help organize modules and provide a naming hierarchy, Python has a concept of packages.
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. Like file system directories, packages are organized hierarchically, and packages may themselves contain subpackages, as well as regular modules.
It’s important to keep in mind that all packages are modules, but not all
modules are packages. Or put another way, packages are just a special kind of
module. Specifically, any module that contains a __path__
attribute is
considered a package.
All modules have a name. Subpackage names are separated from their parent
package name by dots, akin to Python’s standard attribute access syntax. Thus
you might have a module called sys
and a package called email
,
which in turn has a subpackage called email.mime
and a module within
that subpackage called email.mime.text
.
5.2.1. Regular packages¶
Python defines two types of packages, regular packages and namespace packages. Regular
packages are traditional packages as they existed in Python 3.2 and earlier.
A regular package is typically implemented as a directory containing an
__init__.py
file. When a regular package is imported, this
__init__.py
file is implicitly executed, and the objects it defines are
bound to names in the package’s namespace. The __init__.py
file can
contain the same Python code that any other module can contain, and Python
will add some additional attributes to the module when it is imported.
For example, the following file system layout defines a top level parent
package with three subpackages:
parent/
__init__.py
one/
__init__.py
two/
__init__.py
three/
__init__.py
Importing parent.one
will implicitly execute parent/__init__.py
and
parent/one/__init__.py
. Subsequent imports of parent.two
or
parent.three
will execute parent/two/__init__.py
and
parent/three/__init__.py
respectively.
5.2.2. Namespace packages¶
A namespace package is a composite of various portions, where each portion contributes a subpackage to the parent package. Portions may reside in different locations on the file system. Portions may also be found in zip files, on the network, or anywhere else that Python searches during import. Namespace packages may or may not correspond directly to objects on the file system; they may be virtual modules that have no concrete representation.
Namespace packages do not use an ordinary list for their __path__
attribute. They instead use a custom iterable type which will automatically
perform a new search for package portions on the next import attempt within
that package if the path of their parent package (or sys.path
for a
top level package) changes.
With namespace packages, there is no parent/__init__.py
file. In fact,
there may be multiple parent
directories found during import search, where
each one is provided by a different portion. Thus parent/one
may not be
physically located next to parent/two
. In this case, Python will create a
namespace package for the top-level parent
package whenever it or one of
its subpackages is imported.
See also PEP 420 for the namespace package specification.
5.3. Searching¶
To begin the search, Python needs the fully qualified
name of the module (or package, but for the purposes of this discussion, the
difference is immaterial) being imported. This name may come from various
arguments to the import
statement, or from the parameters to the
importlib.import_module()
or __import__()
functions.
This name will be used in various phases of the import search, and it may be
the dotted path to a submodule, e.g. foo.bar.baz
. In this case, Python
first tries to import foo
, then foo.bar
, and finally foo.bar.baz
.
If any of the intermediate imports fail, an ImportError
is raised.
5.3.1. The module cache¶
The first place checked during import search is sys.modules
. This
mapping serves as a cache of all modules that have been previously imported,
including the intermediate paths. So if foo.bar.baz
was previously
imported, sys.modules
will contain entries for foo
, foo.bar
,
and foo.bar.baz
. Each key will have as its value the corresponding module
object.
During import, the module name is looked up in sys.modules
and if
present, the associated value is the module satisfying the import, and the
process completes. However, if the value is None
, then an
ImportError
is raised. If the module name is missing, Python will
continue searching for the module.
sys.modules
is writable. Deleting a key may not destroy the
associated module (as other modules may hold references to it),
but it will invalidate the cache entry for the named module, causing
Python to search anew for the named module upon its next
import. The key can also be assigned to None
, forcing the next import
of the module to result in an ImportError
.
Beware though, as if you keep a reference to the module object,
invalidate its cache entry in sys.modules
, and then re-import the
named module, the two module objects will not be the same. By contrast,
imp.reload()
will reuse the same module object, and simply
reinitialise the module contents by rerunning the module’s code.
5.3.2. Finders and loaders¶
If the named module is not found in sys.modules
, then Python’s import
protocol is invoked to find and load the module. This protocol consists of
two conceptual objects, finders and loaders.
A finder’s job is to determine whether it can find the named module using
whatever strategy it knows about. Objects that implement both of these
interfaces are referred to as importers - they return
themselves when they find that they can load the requested module.
Python includes a number of default finders and importers. The first one knows how to locate built-in modules, and the second knows how to locate frozen modules. A third default finder searches an import path for modules. The import path is a list of locations that may name file system paths or zip files. It can also be extended to search for any locatable resource, such as those identified by URLs.
The import machinery is extensible, so new finders can be added to extend the range and scope of module searching.
Finders do not actually load modules. If they can find the named module, they return a loader, which the import machinery then invokes to load the module and create the corresponding module object.
The following sections describe the protocol for finders and loaders in more detail, including how you can create and register new ones to extend the import machinery.
5.3.3. Import hooks¶
The import machinery is designed to be extensible; the primary mechanism for this are the import hooks. There are two types of import hooks: meta hooks and import path hooks.
Meta hooks are called at the start of import processing, before any other
import processing has occurred, other than sys.modules
cache look up.
This allows meta hooks to override sys.path
processing, frozen
modules, or even built-in modules. Meta hooks are registered by adding new
finder objects to sys.meta_path
, as described below.
Import path hooks are called as part of sys.path
(or
package.__path__
) processing, at the point where their associated path
item is encountered. Import path hooks are registered by adding new callables
to sys.path_hooks
as described below.
5.3.4. The meta path¶
When the named module is not found in sys.modules
, Python next
searches sys.meta_path
, which contains a list of meta path finder
objects. These finders are queried in order to see if they know how to handle
the named module. Meta path finders must implement a method called
find_module()
which takes two arguments, a name and an import path.
The meta path finder can use any strategy it wants to determine whether it can
handle the named module or not.
If the meta path finder knows how to handle the named module, it returns a
loader object. If it cannot handle the named module, it returns None
. If
sys.meta_path
processing reaches the end of its list without returning
a loader, then an ImportError
is raised. Any other exceptions raised
are simply propagated up, aborting the import process.
The find_module()
method of meta path finders is called with two
arguments. The first is the fully qualified name of the module being
imported, for example foo.bar.baz
. The second argument is the path
entries to use for the module search. For top-level modules, the second
argument is None
, but for submodules or subpackages, the second
argument is the value of the parent package’s __path__
attribute. If
the appropriate __path__
attribute cannot be accessed, an
ImportError
is raised.
The meta path may be traversed multiple times for a single import request.
For example, assuming none of the modules involved has already been cached,
importing foo.bar.baz
will first perform a top level import, calling
mpf.find_module("foo", None)
on each meta path finder (mpf
). After
foo
has been imported, foo.bar
will be imported by traversing the
meta path a second time, calling
mpf.find_module("foo.bar", foo.__path__)
. Once foo.bar
has been
imported, the final traversal will call
mpf.find_module("foo.bar.baz", foo.bar.__path__)
.
Some meta path finders only support top level imports. These importers will
always return None
when anything other than None
is passed as the
second argument.
Python’s default sys.meta_path
has three meta path finders, one that
knows how to import built-in modules, one that knows how to import frozen
modules, and one that knows how to import modules from an import path
(i.e. the path based finder).
5.4. Loaders¶
If and when a module loader is found its
load_module()
method is called, with a single
argument, the fully qualified name of the module being imported. This method
has several responsibilities, and should return the module object it has
loaded [2]. If it cannot load the module, it should raise an
ImportError
, although any other exception raised during
load_module()
will be propagated.
In many cases, the finder and loader can be the same object; in such cases the
finder.find_module()
would just return self
.
Loaders must satisfy the following requirements:
If there is an existing module object with the given name in
sys.modules
, the loader must use that existing module. (Otherwise,imp.reload()
will not work correctly.) If the named module does not exist insys.modules
, the loader must create a new module object and add it tosys.modules
.Note that the module must exist in
sys.modules
before the loader executes the module code. This is crucial because the module code may (directly or indirectly) import itself; adding it tosys.modules
beforehand prevents unbounded recursion in the worst case and multiple loading in the best.If loading fails, the loader must remove any modules it has inserted into
sys.modules
, but it must remove only the failing module, and only if the loader itself has loaded it explicitly. Any module already in thesys.modules
cache, and any module that was successfully loaded as a side-effect, must remain in the cache.The loader may set the
__file__
attribute of the module. If set, this attribute’s value must be a string. The loader may opt to leave__file__
unset if it has no semantic meaning (e.g. a module loaded from a database). If__file__
is set, it may also be appropriate to set the__cached__
attribute which is the path to any compiled version of the code (e.g. byte-compiled file). The file does not need to exist to set this attribute; the path can simply point to whether the compiled file would exist (see PEP 3147).The loader may set the
__name__
attribute of the module. While not required, setting this attribute is highly recommended so that therepr()
of the module is more informative.If the module is a package (either regular or namespace), the loader must set the module object’s
__path__
attribute. The value must be iterable, but may be empty if__path__
has no further significance to the loader. If__path__
is not empty, it must produce strings when iterated over. More details on the semantics of__path__
are given below.The
__loader__
attribute must be set to the loader object that loaded the module. This is mostly for introspection and reloading, but can be used for additional loader-specific functionality, for example getting data associated with a loader.The module’s
__package__
attribute should be set. Its value must be a string, but it can be the same value as its__name__
. If the attribute is set toNone
or is missing, the import system will fill it in with a more appropriate value. When the module is a package, its__package__
value should be set to its__name__
. When the module is not a package,__package__
should be set to the empty string for top-level modules, or for submodules, to the parent package’s name. See PEP 366 for further details.This attribute is used instead of
__name__
to calculate explicit relative imports for main modules, as defined in PEP 366.If the module is a Python module (as opposed to a built-in module or a dynamically loaded extension), the loader should execute the module’s code in the module’s global name space (
module.__dict__
).
5.4.1. Module reprs¶
By default, all modules have a usable repr, however depending on the attributes set above, and hooks in the loader, you can more explicitly control the repr of module objects.
Loaders may implement a module_repr()
method which takes a single
argument, the module object. When repr(module)
is called for a module
with a loader supporting this protocol, whatever is returned from
module.__loader__.module_repr(module)
is returned as the module’s repr
without further processing. This return value must be a string.
If the module has no __loader__
attribute, or the loader has no
module_repr()
method, then the module object implementation itself
will craft a default repr using whatever information is available. It will
try to use the module.__name__
, module.__file__
, and
module.__loader__
as input into the repr, with defaults for whatever
information is missing.
Here are the exact rules used:
- If the module has a
__loader__
and that loader has amodule_repr()
method, call it with a single argument, which is the module object. The value returned is used as the module’s repr.- If an exception occurs in
module_repr()
, the exception is caught and discarded, and the calculation of the module’s repr continues as ifmodule_repr()
did not exist.- If the module has a
__file__
attribute, this is used as part of the module’s repr.- If the module has no
__file__
but does have a__loader__
, then the loader’s repr is used as part of the module’s repr.- Otherwise, just use the module’s
__name__
in the repr.
This example, from PEP 420 shows how a loader can craft its own module repr:
class NamespaceLoader:
@classmethod
def module_repr(cls, module):
return "<module '{}' (namespace)>".format(module.__name__)
5.4.2. module.__path__¶
By definition, if a module has an __path__
attribute, it is a package,
regardless of its value.
A package’s __path__
attribute is used during imports of its subpackages.
Within the import machinery, it functions much the same as sys.path
,
i.e. providing a list of locations to search for modules during import.
However, __path__
is typically much more constrained than
sys.path
.
__path__
must be an iterable of strings, but it may be empty.
The same rules used for sys.path
also apply to a package’s
__path__
, and sys.path_hooks
(described below) are
consulted when traversing a package’s __path__
.
A package’s __init__.py
file may set or alter the package’s __path__
attribute, and this was typically the way namespace packages were implemented
prior to PEP 420. With the adoption of PEP 420, namespace packages no
longer need to supply __init__.py
files containing only __path__
manipulation code; the namespace loader automatically sets __path__
correctly for the namespace package.
5.5. The Path Based Finder¶
As mentioned previously, Python comes with several default meta path finders. One of these, called the path based finder, searches an import path, which contains a list of path entries. Each path entry names a location to search for modules.
The path based finder itself doesn’t know how to import anything. Instead, it traverses the individual path entries, associating each of them with a path entry finder that knows how to handle that particular kind of path.
The default set of path entry finders implement all the semantics for finding
modules on the file system, handling special file types such as Python source
code (.py
files), Python byte code (.pyc
and .pyo
files) and
shared libraries (e.g. .so
files). When supported by the zipimport
module in the standard library, the default path entry finders also handle
loading all of these file types (other than shared libraries) from zipfiles.
Path entries need not be limited to file system locations. They can refer to URLs, database queries, or any other location that can be specified as a string.
The path based finder provides additional hooks and protocols so that you can extend and customize the types of searchable path entries. For example, if you wanted to support path entries as network URLs, you could write a hook that implements HTTP semantics to find modules on the web. This hook (a callable) would return a path entry finder supporting the protocol described below, which was then used to get a loader for the module from the web.
A word of warning: this section and the previous both use the term finder,
distinguishing between them by using the terms meta path finder and
path entry finder. These two types of finders are very similar,
support similar protocols, and function in similar ways during the import
process, but it’s important to keep in mind that they are subtly different.
In particular, meta path finders operate at the beginning of the import
process, as keyed off the sys.meta_path
traversal.
By contrast, path entry finders are in a sense an implementation detail
of the path based finder, and in fact, if the path based finder were to be
removed from sys.meta_path
, none of the path entry finder semantics
would be invoked.
5.5.1. Path entry finders¶
The path based finder is responsible for finding and loading Python modules and packages whose location is specified with a string path entry. Most path entries name locations in the file system, but they need not be limited to this.
As a meta path finder, the path based finder implements the
find_module()
protocol previously described, however it exposes
additional hooks that can be used to customize how modules are found and
loaded from the import path.
Three variables are used by the path based finder, sys.path
,
sys.path_hooks
and sys.path_importer_cache
. The __path__
attributes on package objects are also used. These provide additional ways
that the import machinery can be customized.
sys.path
contains a list of strings providing search locations for
modules and packages. It is initialized from the PYTHONPATH
environment variable and various other installation- and
implementation-specific defaults. Entries in sys.path
can name
directories on the file system, zip files, and potentially other “locations”
(see the site
module) that should be searched for modules, such as
URLs, or database queries. Only strings and bytes should be present on
sys.path
; all other data types are ignored. The encoding of bytes
entries is determined by the individual path entry finders.
The path based finder is a meta path finder, so the import
machinery begins the import path search by calling the path
based finder’s find_module()
method as described previously. When
the path
argument to find_module()
is given, it will be a
list of string paths to traverse - typically a package’s __path__
attribute for an import within that package. If the path
argument
is None
, this indicates a top level import and sys.path
is used.
The path based finder iterates over every entry in the search path, and
for each of these, looks for an appropriate path entry finder for the
path entry. Because this can be an expensive operation (e.g. there may be
stat() call overheads for this search), the path based finder maintains
a cache mapping path entries to path entry finders. This cache is maintained
in sys.path_importer_cache
(despite the name, this cache actually
stores finder objects rather than being limited to importer objects).
In this way, the expensive search for a particular path entry
location’s path entry finder need only be done once. User code is
free to remove cache entries from sys.path_importer_cache
forcing
the path based finder to perform the path entry search again [3].
If the path entry is not present in the cache, the path based finder iterates
over every callable in sys.path_hooks
. Each of the path entry
hooks in this list is called with a single argument, the
path entry to be searched. This callable may either return a path
entry finder that can handle the path entry, or it may raise
ImportError
. An ImportError
is used by the path based finder to
signal that the hook cannot find a path entry finder for that
path entry. The exception is ignored and import path
iteration continues. The hook should expect either a string or bytes object;
the encoding of bytes objects is up to the hook (e.g. it may be a file system
encoding, UTF-8, or something else), and if the hook cannot decode the
argument, it should raise ImportError
.
If sys.path_hooks
iteration ends with no path entry finder
being returned, then the path based finder’s find_module()
method
will store None
in sys.path_importer_cache
(to indicate that
there is no finder for this path entry) and return None
, indicating that
this meta path finder could not find the module.
If a path entry finder is returned by one of the path entry
hook callables on sys.path_hooks
, then the following protocol is used
to ask the finder for a module loader, which is then used to load the module.
5.5.2. Path entry finder protocol¶
In order to support imports of modules and initialized packages and also to
contribute portions to namespace packages, path entry finders must implement
the find_loader()
method.
find_loader()
takes one argument, the fully qualified name of the
module being imported. find_loader()
returns a 2-tuple where the
first item is the loader and the second item is a namespace portion.
When the first item (i.e. the loader) is None
, this means that while the
path entry finder does not have a loader for the named module, it knows that the
path entry contributes to a namespace portion for the named module. This will
almost always be the case where Python is asked to import a namespace package
that has no physical presence on the file system. When a path entry finder
returns None
for the loader, the second item of the 2-tuple return value
must be a sequence, although it can be empty.
If find_loader()
returns a non-None
loader value, the portion is
ignored and the loader is returned from the path based finder, terminating
the search through the path entries.
For backwards compatibility with other implementations of the import
protocol, many path entry finders also support the same,
traditional find_module()
method that meta path finders support.
However path entry finder find_module()
methods are never called
with a path
argument (they are expected to record the appropriate
path information from the initial call to the path hook).
The find_module()
method on path entry finders is deprecated,
as it does not allow the path entry finder to contribute portions to
namespace packages. Instead path entry finders should implement the
find_loader()
method as described above. If it exists on the path
entry finder, the import system will always call find_loader()
in preference to find_module()
.
5.6. Replacing the standard import system¶
The most reliable mechanism for replacing the entire import system is to
delete the default contents of sys.meta_path
, replacing them
entirely with a custom meta path hook.
If it is acceptable to only alter the behaviour of import statements
without affecting other APIs that access the import system, then replacing
the builtin __import__()
function may be sufficient. This technique
may also be employed at the module level to only alter the behaviour of
import statements within that module.
To selectively prevent import of some modules from a hook early on the
meta path (rather than disabling the standard import system entirely),
it is sufficient to raise ImportError
directly from
find_module()
instead of returning None
. The latter indicates
that the meta path search should continue. while raising an exception
terminates it immediately.
5.7. Open issues¶
XXX It would be really nice to have a diagram.
XXX * (import_machinery.rst) how about a section devoted just to the attributes of modules and packages, perhaps expanding upon or supplanting the related entries in the data model reference page?
XXX runpy, pkgutil, et al in the library manual should all get “See Also” links at the top pointing to the new import system section.
5.8. References¶
The import machinery has evolved considerably since Python’s early days. The original specification for packages is still available to read, although some details have changed since the writing of that document.
The original specification for sys.meta_path
was PEP 302, with
subsequent extension in PEP 420.
PEP 420 introduced namespace packages for
Python 3.3. PEP 420 also introduced the find_loader()
protocol as an
alternative to find_module()
.
PEP 366 describes the addition of the __package__
attribute for
explicit relative imports in main modules.
PEP 328 introduced absolute and explicit relative imports and initially
proposed __name__
for semantics PEP 366 would eventually specify for
__package__
.
PEP 338 defines executing modules as scripts.
Footnotes
[1] | See types.ModuleType . |
[2] | The importlib implementation avoids using the return value
directly. Instead, it gets the module object by looking the module name up
in sys.modules . The indirect effect of this is that an imported
module may replace itself in sys.modules . This is
implementation-specific behavior that is not guaranteed to work in other
Python implementations. |
[3] | In legacy code, it is possible to find instances of
imp.NullImporter in the sys.path_importer_cache . It
is recommended that code be changed to use None instead. See
Porting Python code for more details. |