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How can I start my Python code, from the code itself?
I have already learned how to tell the Python interpreter to run my code, but the standard approach feels a little unsatisfactory. I want the user experience to be that my program starts with the actual .py file, not with a Python interpreter. It's okay if a Python interpreter needs to be installed on the system already, but how can I make my Python program just run by double-clicking the file, or by typing the filename by itself (without python or py) at the command line?
1 answer
The following users marked this post as Works for me:
| User | Comment | Date |
|---|---|---|
| Karl Knechtel | (no comment) | Jun 14, 2024 at 10:25 |
Mac/Linux
In this environment, Python scripts can be given a shebang line that tells the operating system that the file is a script, and what interpreter (i.e., Python) to use for it. Because # starts a comment in Python, Python itself doesn't need any special handling for this - it's just ignored anyway. Of course, the file also needs to be marked as executable (chmod +x).
Keep in mind that without a shebang, the operating system will typically try to treat the script as a shell script, which will fail completely (probably with many errors). The operating system doesn't care about a .py extension on the file name - only the shebang matters.
It's common to use something like #!/usr/bin/env python, to allow the operating system to find Python. But sometimes it's necessary to give the path to a specific Python to make the script work properly.
It's also common for such scripts to name the file without a .py extension (so just myscript), and to not use a if __name__ == '__main__': guard. That's because a script like this will normally only be used as a script and should not ever be imported from somewhere else - so it's completely unnecessary and inelegant to add those things.
Complete example
How the script is named, and the permissions:
$ ls
myscript
$ stat -c '%A' myscript
-rwxrwxr-x
What's in it:
$ cat myscript
#!/usr/bin/env python
def my_function():
print("Test")
my_function()
How we run it, and what happens:
$ ./myscript
Test
Windows
Windows doesn't care about shebangs generally, and doesn't have a concept of executable text files. However, it does have a concept of file associations - so we can tell Windows to use Python (via a specific path to python, perhaps something likeC:\Program Files\Python 3.12\python.exe) to open any file with a .py file name extension when it's double-clicked.
But better yet, we can tell it to use the Python Launcher for Windows. There's only one of these even if you install multiple versions of Python, and it's in a single, consistent place -C:\Windows\py.exe (assuming Python was installed "for all users"). More importantly, this py program tries to read and understand shebang lines - simply write the script as if it would run on a Mac or Linux system, and let py do the rest.
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