CODEWITHSUNDEEP
python-3.ximport
Godrebh
Godrebh

Reputation: 564

Importing one module from different other modules only executes it once. Why?

I am confused about some behavior of Python. I always thought importing a module basically meant executing it. (Like they say here: Does python execute imports on importation) So I created three simple scripts to test something:

main.py

import config
print(config.a)

config.a += 1
print(config.a)

import test
print(config.a)

config.py

def get_a():
    print("get_a is called")
    return 1
a = get_a()

test.py

import config
print(config.a)
config.a += 1

The output when running main.py is:

get_a is called
1
2
2
3

Now I am confused because I expected get_a() to be called twice, once from main.py and once from test.py. Can someone please explain why it is not? What if I really wanted to import config a second time, like it was in the beginning with a=1?

(Fortunately, for my project this behavior is exactly what I wanted, because get_a() corresponds to a function, which reads lots of data from a database and of course I only want to read it once, but it should be accessible from multiple modules.)

Upvotes: 0

Views: 36

Answers (1)

multivac
multivac

Reputation: 146

Because the config module is already loaded so there's no need to 'run' it anymore, just return the loaded instance.

Some standard library modules make use of this, from example random. It creates an object of class Random on first import and reuses it when it gets imported again. A comment on the module reads:

# Create one instance, seeded from current time, and export its methods
# as module-level functions.  The functions share state across all uses
#(both in the user's code and in the Python libraries), but that's fine
# for most programs and is easier for the casual user than making them
# instantiate their own Random() instance.

Upvotes: 1

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