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This is apparently the same question as this (S.O. didn't find it until later). Am investigating duping. However, the title line alone makes this a useful question.

In the following (two) chunks of code, why don't I get the updated value when I import?

Two files: import.py and export.py Here is export.py:

i=12345

def change_i(x):
  global i
  i=x
  print "export changed i to {0}".format(repr(i))

Here is import.py:

from export import i, change_i

print "Import gets i={0}".format(repr(i))
change_i(5)
print "Import has i={0}".format(repr(i))

Here is the result:

> python import.py 
Import gets i=12345
export changed i to 5
Import has i=12345

Why isn't import.py getting the new value 5????

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  • Yes. But S.O. didn't find that question until you did. So yes it's a duplicate but I would maintain that the secondary search term is useful. There is absolutely no indication that these are the same question from the titles. Commented May 8, 2017 at 19:06

1 Answer 1

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NOPE. TURNS OUT THIS IS A LIE/MISCONCEPTION.

When you import, you import the current value of the imported symbol and add it to your namespace as defined. You are not importing a reference, you are importing a value.

Thus, to get the updated value of i, you must import a variable that holds a reference to that symbol.

In other words, importing is NOT like an import in JAVA, external declaration in C/C++ or even a use clause in PERL.

Rather, the following statement in Python:

from some_other_module import a as x

is more like the following code in K&R C:

extern int a; /* import from the EXTERN file */

int x = a;

(caveat: in the Python case, "a" and "x" are essentially a reference to the actual value: you're not copying the INT, you're copying the reference address)

This is sneakily defined in the Python language reference: the use of object instead of symbol. I would suggest that the Python language reference make this more clear and less legal.

The from form does not bind the module name: it goes through the list of identifiers, looks each one of them up in the module found in step (1), and binds the name in the local namespace to the object thus found.

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8 Comments

Now try the same with a list. Assign a list to i and modify change_i to append or otherwise modify i without changing what i is referring to.
You are just repeating my statement from the beginning of my answer. What's your point?
Actually, you do import a reference to the same object that is in the other module. No lie, no copy. The thing is, i=x rebinds the i in your module namespace to a different object.
@MarkGerolimatos my point is that you have a misconception about how Python references and import system work and so your answer is wrong. You didn't try it with a list, did you? The list gets updated and you can see it changing inside import.py, too.
Python imports a reference to the object, which is referred to by the variable you're trying to import. Yes, rebinding it to some other object inside the module won't change the reference you imported earlier. Still, technically, Python imports a reference to a value, and this means that your first paragraph is misleading.
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