Often, a Python program is run by naming a .py file on the command line:
$ python my_program.py
You can also create a directory or zipfile full of code, and include a __main__.py. Then you can simply name the directory or zipfile on the command line, and it executes the __main__.py automatically:
$ python my_program_dir
$ python my_program.zip
# Or, if the program is accessible as a module
$ python -m my_program
You'll have to decide for yourself whether your application could benefit from being executed like this.
Note that a __main__ module usually doesn't come from a __main__.py file. It can, but it usually doesn't. When you run a script like python my_program.py, the script will run as the __main__ module instead of the my_program module. This also happens for modules run as python -m my_module, or in several other ways.
If you saw the name __main__ in an error message, that doesn't necessarily mean you should be looking for a __main__.py file.
Often, a Python program is run by naming a .py file on the command line:
$ python my_program.py
You can also create a directory or zipfile full of code, and include a __main__.py. Then you can simply name the directory or zipfile on the command line, and it executes the __main__.py automatically:
$ python my_program_dir
$ python my_program.zip
# Or, if the program is accessible as a module
$ python -m my_program
You'll have to decide for yourself whether your application could benefit from being executed like this.
Note that a __main__ module usually doesn't come from a __main__.py file. It can, but it usually doesn't. When you run a script like python my_program.py, the script will run as the __main__ module instead of the my_program module. This also happens for modules run as python -m my_module, or in several other ways.
If you saw the name __main__ in an error message, that doesn't necessarily mean you should be looking for a __main__.py file.
What is the __main__.py file for?
When creating a Python module, it is common to make the module execute some functionality (usually contained in a main function) when run as the entry point of the program. This is typically done with the following common idiom placed at the bottom of most Python files:
if __name__ == '__main__':
# execute only if run as the entry point into the program
main()
You can get the same semantics for a Python package with __main__.py, which might have the following structure:
.
└── demo
├── __init__.py
└── __main__.py
To see this, paste the below into a Python 3 shell:
from pathlib import Path
demo = Path.cwd() / 'demo'
demo.mkdir()
(demo / '__init__.py').write_text("""
print('demo/__init__.py executed')
def main():
print('main() executed')
""")
(demo / '__main__.py').write_text("""
print('demo/__main__.py executed')
from demo import main
main()
""")
We can treat demo as a package and actually import it, which executes the top-level code in the __init__.py (but not the main function):
>>> import demo
demo/__init__.py executed
When we use the package as the entry point to the program, we perform the code in the __main__.py, which imports the __init__.py first:
$ python -m demo
demo/__init__.py executed
demo/__main__.py executed
main() executed
You can derive this from the documentation. The documentation says:
__main__— Top-level script environment
'__main__'is the name of the scope in which top-level code executes. A module’s__name__is set equal to'__main__'when read from standard input, a script, or from an interactive prompt.A module can discover whether or not it is running in the main scope by checking its own
__name__, which allows a common idiom for conditionally executing code in a module when it is run as a script or withpython -mbut not when it is imported:if __name__ == '__main__': # execute only if run as a script main()For a package, the same effect can be achieved by including a
__main__.pymodule, the contents of which will be executed when the module is run with-m.
Zipped
You can also zip up this directory, including the __main__.py, into a single file and run it from the command line like this - but note that zipped packages can't execute sub-packages or submodules as the entry point:
from pathlib import Path
demo = Path.cwd() / 'demo2'
demo.mkdir()
(demo / '__init__.py').write_text("""
print('demo2/__init__.py executed')
def main():
print('main() executed')
""")
(demo / '__main__.py').write_text("""
print('demo2/__main__.py executed')
from __init__ import main
main()
""")
Note the subtle change - we are importing main from __init__ instead of demo2 - this zipped directory is not being treated as a package, but as a directory of scripts. So it must be used without the -m flag.
Particularly relevant to the question - zipapp causes the zipped directory to execute the __main__.py by default - and it is executed first, before __init__.py:
$ python -m zipapp demo2 -o demo2zip
$ python demo2zip
demo2/__main__.py executed
demo2/__init__.py executed
main() executed
Note again, this zipped directory is not a package - you cannot import it either.
How to run a Python script from the main script?
How to run main.py?
Python IDLE: Run main? - Stack Overflow
what is the simplest way to run a a python script?
I'm currently working on a project to detect tools in a box using Yolov7, OpenCV etc. on my Jetson Nano, and one of the features is to see the dot matrix digits on the weighing scale in the box to convert it into a string which will send it into a database.
Right now I have the main script that detects the tools called detect.py and I have made a separate script to see the digits called code.py. But the problem I'm facing now is I'm struggling to integrate both codes together and I wonder if there is a way for me to run code.py once I run detect.py so they both run at the same time.
Would appreciate the assistance. Sorry if the description is too vague!
I'm doing a course on Python. I have a main.py file. If I open the parent folder and try to run the file, it doesn't work. Why's that?
I get an error like this "ImportError: No module named tkinter"
I'm sure there's a basic fundamental I'm not understanding.
Thanks.
The if condition on __name__ == '__main__' is meant for code to be run when your module is executed directly and not run when it is imported. There is really no such concept of "main" as e.g. in Java. As Python is interpreted, all lines of code are read and executed when importing/running the module.
Python provides the __name__ mechanism for you to distinguish the import case from the case when you run your module as a script i.e. python mymodule.py. In this second case __name__ will have the value '__main__'
If you want a main() that you can run, simply write:
def main():
do_stuff()
more_stuff()
if __name__ == '__main__':
main()
If you import something it's not main. You need to run it from the menu or as an argument when you start idle.
i have this script that chatgpt made for me, i want to test it out but i am new to coding and im not sure how to run it.
i tried googling it but many of them go into way to deep detail and have just walls of text that for someone with dyslexia is very hard to read. i have python and pip installed and they are up to date along with notepad++
import os
def rename_files_in_folder(folder_path, prefix, exclude_filename=None):
# Get a list of all files in the folder
files = os.listdir(folder_path)
# Rename each file with a prefix and number, excluding the specified file
for i, filename in enumerate(files):
if filename == exclude_filename:
continue
new_filename = f"{prefix}{i + 1}{os.path.splitext(filename)[1]}"
os.rename(os.path.join(folder_path, filename), os.path.join(folder_path, new_filename))
# Example usage
rename_files_in_folder("/path/to/folder", "new_file_", exclude_filename="file_to_exclude.txt")