How to send an argument to another file in python 3

Question

So, I have been looking at other threads on this topic but they do not use the Module version of passing the argument to the other file, if so wasn't explained clearly enough for me to understand.

I have tried just to show I'm not just asking:

#MoneyCounter.py
import Password

enter = False
Password.system(enter)

def start(check):
    if check == True:
        main()
    elif check == False:
        print("Critical Error occured")
        exit()

And my other file

#Password.py
import MoneyCounter

def system(check):
    if check == False:
        password() #This goes to password def not mentioned in this code
    elif check == True:
         MoneyCounter.start(check)

The error I am getting is the Module Password has no attribute system

What do you mean by “passing an argument to a file”? Also, your issue here is you cannot import module A from module B if module A also imports module B (aka. import loop) — Valentin Lorentz, May 14 '16 at 18:06
You named the argument `check` but use `enter` in the function.. — Martijn Pieters, May 14 '16 at 18:07
Well, you *can* do a circular import, but you need to know what you're doing. — Ignacio Vazquez-Abrams, May 14 '16 at 18:07
Not related to your problem, but `exit` should only be used in interactive python (because of side-effects involving `stdin`). You should use `sys.exit()` instead. — cdarke, May 14 '16 at 18:13
thanks cdark. But how would i be able to do the circular import which is what i would like to do since i want to send arguments back and forth between the files — Oliver Carlton, May 14 '16 at 18:16

score 2 · Answer 1 · answered May 14 '16 at 18:12

2

The error I am getting is the Module Password has no attribute system

Of course it doesn't. The definition doesn't exist by the time the line of code is executed, since execution of the first file got interrupted by the import.

Either refactor or reorder your code so that the name isn't accessed until it exists, or remove the requirement that each module has for the other.

answered May 14 '16 at 18:12

Ignacio Vazquez-Abrams

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This answer is correct, but I would put more emphasis on the refactoring option. Having circular imports is just asking for trouble and odd errors (like this). – skrrgwasme May 14 '16 at 19:00

Marcus Müller · Answer 2 · 2016-05-14T19:09:25.320

0

Your problem here is circular dependency/imports.

An import statement really executes the code in the imported file; that is, all the statements are executed, everything that is defed in the imported file gets defined etc. imports get imported, too.

So what's happening is this:

you run

$ python MoneyCounter.py

Python reads MoneyCounter.py, executes its first statement: import Password
Python reads Password.py, and executes its first statement: import MoneyCounter.py
Python reads MoneyCounter.py, this time it encounters import Password, but already has password in its list of known names; so it continues to

enter=False; Password.system(enter).

Now, Python already has a Password in its name lookup dictionary: the half-imported Password. In that, def system… hasn't happened yet, so Password.system is still unknown.

Generally, your architecture is questionable. Why would the Password "utility" module call into your "master" business logic? Better not do that, but write code that actually checks the return value of Password.system and acts based on that in your MoneyCounter.py.

edited May 14 '16 at 19:09

answered May 14 '16 at 18:12

Marcus Müller

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Step 4 is wrong given step 1, since the main script is always `__main__` regardless of what filename it has. – Ignacio Vazquez-Abrams May 14 '16 at 18:14
@IgnacioVazquez-Abrams I must admit I'm confused: There's no usage of `__main__` anywhere? – Marcus Müller May 14 '16 at 18:15
1

Right, which means that `Password` would be importing a *second* copy of MoneyCounter.py as `MoneyCounter` in addition to the copy as `__main__`. Trap for young players. – Ignacio Vazquez-Abrams May 14 '16 at 18:18
aaaah yes indeed, and the point is that upon importing that, Python would encounter `import Password`; now, already having a name `Password` in its dict, it doesn't `import Password`? – Marcus Müller May 14 '16 at 18:40
Correct, it would use the reference it already has. Which doesn't yet have the attribute in question. – Ignacio Vazquez-Abrams May 14 '16 at 18:42

cdarke · Answer 3 · 2016-05-14T19:22:32.477

On the assumption that MoneyCounter.py is the entry point (the name you run from the command-line), then I suggest you replace this:

enter = False
Password.system(enter)

with this:

if __name__ == "__main__":
    enter = False
    Password.system(enter)

That will only be executed from the entry-point (which is called __main__) and not when it is imported. However, you should reconsider your design.

Edit:

name is a reference to the text name of the current module. With modules that are explicitly imported the name is taken from the filename, but the entry-point (where the program starts) is always called __main__. So we can always test if we are running as an imported module or as an entry-point.

This test is extremely common in python. This way we can provide additional functionality depending on whether we run a module as a program or import it.

can you please explain what __name__ and "__main__" mean? With the underscores — Oliver Carlton, May 14 '16 at 18:34

How to send an argument to another file in python 3

3 Answers3