How does python load global variables?

Question

[Couldn't find exactly what I wanted. Apologies if this is a duplicate question.]

In this code:

import heavy_function

_data = heavy_function()

def foo():
    print _data

def bar():
    print 'Call no 1'
    foo()
    print 'Call no 2'
    foo()

bar()

If I am calling

bar()

as it is shown in the code, or if I call it from other module by importing it, does the

_data

get loaded each time the function calls it? or does it get loaded once and be stored for further use?

Answer 1

As soon as you load this module your heavy_function() gets loaded and store it's value into _data and this _data gets stored in globals(). And then it doesn't matter how many time you use that same object gets passed over.

For your understanding here I'm simultaing the function value just by giving a list [1, 2, 3], here I'm checking the object identity by comparing foo() return value with _data and refcount to show that it not getting referenced more than once.

#import heavy_function
import sys
_data = [1, 2, 3] # heavy_function()

def foo():
    print _data

def bar():
    print 'Call no 1'
    value1 = foo()
    print sys.getrefcount(_data)
    print 'Call no 2'
    value2 = foo()
    print sys.getrefcount(_data)
    print 'call no 3'
    value3 = foo()
    print sys.getrefcount(_data)

    print 'Check referenced values..'
    print value1 is value2 

bar()

Output:

Call no 1
[1, 2, 3]
2
Call no 2
[1, 2, 3]
2
call no 3
[1, 2, 3]
2
Check referenced values..
True

Here if you see even after the third call the same object is getting returned from foo and this print value1 is value2 one helps you to identify that.

Answer 2

It's loaded once, then the returned value is referenced twice

Answer 3

_data will be assigned the return value of heavy_function() only once (when the module is first imported or run). After that it will simply be accessed.

You should be careful not to try to assign it inside a function though without specifying first that it is a global by using the global keyword.