Why is Python saying modules are imported when they are not?

Question

Python 3.6.5

Using this answer as a guide, I attempted to see whether some modules, such as math were imported.

But Python tells me they are all imported when they are not.

>>> import sys
>>> 'math' in sys.modules
True
>>> 'math' not in sys.modules
False
>>> math.pi
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'math' is not defined
>>> import math
>>> 'math' in sys.modules
True
>>> math.pi
3.141592653589793

Asked By: EmmaV

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Source

Answer 1

to explain this, let’s define this function:

def import_math():
    import math

import_math()

the above function will import the module math, but only in its local scope, anyone that tries to reference math outside of it will get a name error, because math is not defined in the global scope.

any module that is imported is saved into sys.modules so a call to check

import_math()
print("math" in sys.modules)

will print True, because sys.modules caches any module that is loaded anywhere, whether or not it was available in the global scope, a very simple way to define math in the global scope would then to

import_math()
math = sys.modules["math"]

which will convert it from being only in sys.modules to being in the global scope, this is just equivalent to

import math

which defines a variable math in the global scope that points to the module math.

now if you want to see whether "math" exists in the global scope is to check if it is in the global scope directly.

print("math" in globals())
print("math" in locals())

which will print false if "math" wasn’t imported into the global or local scope and is therefore inaccessable.

Answered By: Ahmed AEK

Why is Python saying modules are imported when they are not?

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