Why is Hello printed only two times?

Question

main.py

#main.py
import main
print('Hello')

Output:

Hello
Hello

I believe that when it comes to the line import main, at that time, main is registered in sys.modules and hence the import statement of another script - which I believe, is not a part of __main__ - is not executed. Can someone please tell me whether I understand it correctly? If not, please give an explanation.

So..are you trying to do some kind of recursive printing of `hello` here? — Roland Deschain, Jun 10 '20 at 06:30
Does this answer your question? [Why is Python running my module when I import it, and how do I stop it?](https://stackoverflow.com/questions/6523791/why-is-python-running-my-module-when-i-import-it-and-how-do-i-stop-it) — Maghil vannan, Jun 10 '20 at 06:30
OK, only a guess here, but I would say you can only import stuff once... — Roland Deschain, Jun 10 '20 at 06:31

Klaus D. · Accepted Answer · 2020-06-10T06:41:20.020

Let's add a little debugging output:

import sys
print([key for key in sys.modules.keys() if 'main' in key])
import main

It prints:

['__main__']
['__main__', 'main']

Why is that?

If you run a module it will not be added as its modules name to sys.modules. Instead it will always be __main__.

If you then import the module by its name (main). That name is not present in sys.modules and as the result the module will be imported again, its code executed and the modules stored in sys.modules under its name.

On executing main.py it will print ['__main__'] and on the re-import it will print both module names: ['__main__', 'main'].

This implies one rule: try not to import the module you are running anywhere in your code.

score 1 · Answer 2 · answered Jun 10 '20 at 06:35

1

It only prints it twice because a module is only actually loaded once. This prevents possible unbound recursion. So your print statement gets executed once by the imported module and once by the main program.

answered Jun 10 '20 at 06:35

Mario Camilleri

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score 0 · Answer 3 · answered Jun 10 '20 at 06:31

0

Since you're importing main inside main the print statement is executed twice,thats how python works

answered Jun 10 '20 at 06:31

Maghil vannan

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I may be wrong, but I feel like the question is not WHY it is printing twice, but it is ONLY printing twice. Thus, OP knows about this feature – Roland Deschain Jun 10 '20 at 06:33
but the code can only print hello two times,there are no other print statements – Maghil vannan Jun 10 '20 at 06:35
1

I get it now he asks why it isn't calling main again and again,the question should be edited to why it is not looping then – Maghil vannan Jun 10 '20 at 06:38

Why is Hello printed only two times?

3 Answers3