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Understanding the `if __name__ == '__main__':` idiom
I've seen many examples of Python scripts that include a line that says:
if __name__ == '__main__':
Sometimes the following block contains a bunch of code, but other times it just makes a single function call (like main()), or some variant along the lines of
import sys
# other stuff here...
def main(args):
...
if __name__ == '__main__':
sys.exit(main(sys.argv[1:]))
I can see that this must be checking the value of some __name__ global variable - but that isn't defined anywhere in the code.
Why would there be such a variable, and why might it be equal to __main__? What is the purpose of doing this check, and why might I include it in my own code?
1 answer
The following users marked this post as Works for me:
| User | Comment | Date |
|---|---|---|
| Karl Knechtel | (no comment) | Jun 15, 2024 at 09:37 |
Whenever Python loads code from a .py file, that code gets its own namespace for global variables - so they're not truly global, but per-file values. (When you look at an imported module "from outside", those global variables are the attributes of the module object. Thus, when module.py has value = 1 at top level, code which does import module can then see module.value.)
Python also pre-defines a few of these names. In particular, it sets __name__. When the file was imported, the value for __name__ will be the module name. But when the file is run as a script, __name__ is set to the special value '__main__'.
Therefore, we can simply write an if statement that checks for this value, to know whether the file was imported or not. This allows the file to do specific things only when it's used as a script. For example, older standard library modules used this to contain ad-hoc testing code - you could run a module using python -m to see the test results, but importing the module would skip running the tests.
Sometimes people talk about this idiom as if it defined an "entry point" for the script. But this is not accurate. The code still runs from the top to the bottom (defining global variables and classes and functions along the way). However, wrapping up the code this way (especially if it's organized using a main function) is Pythonic: it clearly signals intent to the reader (this code may be run directly) and keeps things neatly organized.
There are a few more interesting situations that can arise due to how this system works. (Credit to Mr Fooz' answer on Stack Overflow for pointing these out. The explanation given here is my own.)
Note that this allows code in the "main" script to import itself once - the module cache will contain two separate modules created from the same code: one named __main__, and one named according to the file. This can somewhat complicate debugging if you have an issue caused by a circular import.
Also, notice that because of how Python handles command-line arguments, you cannot detect whether the code is being imported this way. Further, command-line arguments that make sense for one script may not make sense for another, so letting the "script" code run on import can cause serious problems besides just wasting time.
Imports can also happen in somewhat unexpected circumstances. For example, if imported.py defines a class, and importer.py tries to unpickle an instance of that class, that will trigger an import of imported.py. For this reason, some people prefer to include this sort of "guard" even when the file is only ever intended to be used as a script.
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