Python: Why do my module variables lose their content?

Question

Please have a look at that "module":

"""Module a"""

a = None
b = None


def gna():
    global a
    if a is None:
        global b
        a = 7
        b = "b"
    print("in a.py: a={}".format(a))
    print("in a.py: b={}".format(b))

I would have thought that calling gna() from another module would initialise the variables:

"""Module b"""

from a import a, b, gna

print("in b.py: a={}".format(a))
print("in b.py: b={}".format(b))

gna()

print("in b.py: a={}".format(a))
print("in b.py: b={}".format(b))

But:

% python3 b.py
in b.py: a=None
in b.py: b=None
in a.py: a=7
in a.py: b=b
in b.py: a=None
in b.py: b=None

And I don't really get why a and b are None after calling gna...

Answer 1

Once you import a name into a module, the name becomes local. You should import module a instead of importing variables a and b from module a so that module b would be able to access the same references to variables a and b whose values the function gna modifies:

"""Module b"""

import a

print("in b.py: a={}".format(a.a))
print("in b.py: b={}".format(a.b))

a.gna()

print("in b.py: a={}".format(a.a))
print("in b.py: b={}".format(a.b))