PEP 3122 – Delineation of the main module
- Brett Cannon
- Standards Track
This PEP has been rejected. Guido views running scripts within a package as an anti-pattern .
Because of how name resolution works for relative imports in a world
where PEP 328 is implemented, the ability to execute modules within a
package ceases being possible. This failing stems from the fact that
the module being executed as the “main” module replaces its
__name__ attribute with
"__main__" instead of leaving it as
the absolute name of the module. This breaks import’s ability
to resolve relative imports from the main module into absolute names.
In order to resolve this issue, this PEP proposes to change how the
main module is delineated. By leaving the
__name__ attribute in
a module alone and setting
sys.main to the name of the main
module this will allow at least some instances of executing a module
within a package that uses relative imports.
This PEP does not address the idea of introducing a module-level function that is automatically executed like PEP 299 proposes.
With the introduction of PEP 328, relative imports became dependent on
__name__ attribute of the module performing the import. This
is because the use of dots in a relative import are used to strip away
parts of the calling module’s name to calculate where in the package
hierarchy an import should fall (prior to PEP 328 relative
imports could fail and would fall back on absolute imports which had a
chance of succeeding).
For instance, consider the import
from .. import spam made from the
bacon.ham.beans module (
bacon.ham.beans is not a package
itself, i.e., does not define
__path__). Name resolution of the
relative import takes the caller’s name (
on dots, and then slices off the last n parts based on the level
(which is 2). In this example both
beans are dropped
spam is joined with what is left (
bacon). This leads to
the proper import of the module
This reliance on the
__name__ attribute of a module when handling
relative imports becomes an issue when executing a script within a
package. Because the executing script has its name set to
'__main__', import cannot resolve any relative imports, leading to
For example, assume we have a package named
bacon with an
__init__.py file containing:
from . import spam
Also create a module named
spam within the
bacon package (it
can be an empty file). Now if you try to execute the
package (either through
python bacon/__init__.py or
python -m bacon) you will get an
ImportError about trying to
do a relative import from within a non-package. Obviously the import
is valid, but because of the setting of
import thinks that
bacon/__init__.py is not in a package since no
dots exist in
__name__. To see how the algorithm works in more
importlib.Import._resolve_name() in the sandbox
Currently a work-around is to remove all relative imports in the module being executed and make them absolute. This is unfortunate, though, as one should not be required to use a specific type of resource in order to make a module in a package be able to be executed.
The solution to the problem is to not change the value of
in modules. But there still needs to be a way to let executing code
know it is being executed as a script. This is handled with a new
attribute in the
sys module named
When a module is being executed as a script,
sys.main will be set
to the name of the module. This changes the current idiom of:
if __name__ == '__main__': ...
import sys if __name__ == sys.main: ...
The newly proposed solution does introduce an added line of boilerplate which is a module import. But as the solution does not introduce a new built-in or module attribute (as discussed in Rejected Ideas) it has been deemed worth the extra line.
Another issue with the proposed solution (which also applies to all
rejected ideas as well) is that it does not directly solve the problem
of discovering the name of a file. Consider
By the file name alone it is not obvious whether
bacon is a
package. In order to properly find this out both the current
direction must exist on
sys.path as well as
But this is the simple example. Consider
python ../spam.py. From
the file name alone it is not at all clear if
spam.py is in a
package or not. One possible solution is to find out what the
absolute name of
.., check if a file named
and then look if the directory is on
sys.path. If it is not, then
continue to walk up the directory until no more
are found or the directory is found on
This could potentially be an expensive process. If the package depth
happens to be deep then it could require a large amount of disk access
to discover where the package is anchored on
sys.path, if at all.
The stat calls alone can be expensive if the file system the executed
script is on is something like NFS.
Because of these issues, only when the
-m command-line argument
(introduced by PEP 338) is used will
__name__ be set. Otherwise
the fallback semantics of setting
sys.main will still be set to the proper value,
regardless of what
__name__ is set to.
-m option is used,
sys.main will be set to the
argument passed in.
sys.argv will be adjusted as it is currently.
Then the equivalent of
__import__(self.main) will occur. This
differs from current semantics as the
runpy module fetches the
code object for the file specified by the module name in order to
__name__ and other attributes. This is no longer
needed as import can perform its normal operation in this situation.
If a file name is specified, then
sys.main will be set to
"__main__". The specified file will then be read and have a code
object created and then be executed with
__name__ set to
"__main__". This mirrors current semantics.
In order for Python 2.6 to be able to support both the current
semantics and the proposed semantics,
sys.main will always be set
"__main__". Otherwise no change will occur for Python 2.6.
This unfortunately means that no benefit from this change will occur
in Python 2.6, but it maximizes compatibility for code that is to
work as much as possible with 2.6 and 3.0.
To help transition to the new idiom, 2to3  will gain a rule to
transform the current
if __name__ == '__main__': ... idiom to the
new one. This will not help with code that checks
outside of the idiom, though.
A counter-proposal to introduce a built-in named
The value of the built-in would be the name of the module being
executed (just like the proposed
sys.main). This would lead to a
new idiom of:
if __name__ == __main__: ...
A drawback is that the syntactic difference is subtle; the dropping of quotes around “__main__”. Some believe that for existing Python programmers bugs will be introduced where the quotation marks will be put on by accident. But one could argue that the bug would be discovered quickly through testing as it is a very shallow bug.
While the name of built-in could obviously be different (e.g.,
main) the other drawback is that it introduces a new built-in.
With a simple solution such as
sys.main being possible without
adding another built-in to Python, this proposal was rejected.
__main__ module attribute
Another proposal was to add a
__main__ attribute to every module.
For the one that was executing as the main module, the attribute would
have a true value while all other modules had a false value. This has
a nice consequence of simplify the main module idiom to:
if __main__: ...
The drawback was the introduction of a new module attribute. It also required more integration with the import machinery than the proposed solution.
__file__ instead of
Any of the proposals could be changed to use the
attribute on modules instead of
__name__, including the current
semantics. The problem with this is that with the proposed solutions
there is the issue of modules having no
__file__ attribute defined
or having the same value as other modules.
The problem that comes up with the current semantics is you still have to try to resolve the file path to a module name for the import to work.
Special string subclass for
__name__ that overrides
One proposal was to define a subclass of
str that overrode the
__eq__ method so that it would compare equal to
well as the actual name of the module. In all other respects the
subclass would be the same as
This was rejected as it seemed like too much of a hack.
- 2to3 tool (http://svn.python.org/view/sandbox/trunk/2to3/) [ViewVC]
- importlib (http://svn.python.org/view/sandbox/trunk/import_in_py/importlib.py?view=markup) [ViewVC]
- Python-Dev email: “PEP to change how the main module is delineated” (https://mail.python.org/pipermail/python-3000/2007-April/006793.html)
This document has been placed in the public domain.
Last modified: 2022-01-21 11:03:51 GMT