Understanding global variable in Python

Question

I came across a strange issue in Python when using global variables. I have two modules(files):`mod1.py` and `mod2.py` `mod1` tries to modify the global variable `var` defined in `mod2`. But the `var` in mod2 and `var` in `mod` seems to be two different things. Thus, the result shows that such modification does not work. Here is the code: ``` #code for mod2.py global var var = 1 def fun_of_mod2(): print var #code for mod1.py from mod2 import var,fun_of_mod2 global var #commenting out this line yields the same result var = 2 #I want to modify the value of var defined in mod2 fun_of_mod2() #but it prints: 1 instead of 2. Modification failed :-( ``` Any hint on why this happens? And how can I modify the value of `val` defined in `mod2` in `mod1`? Thanks

Accepted Answer

When you import `var` into `mod1`:

```
from mod2 import var,fun_of_mod2
```

You are giving it the name `var` in mod1's namespace. It is as if you did this:

```
import mod2
var = mod2.var
fun_of_mod2 = mod2.fun_of_mod2
del mod2
```

In other words, there are now two names for the value, `mod1.var` and `mod2.var`. They are the same at first, but when you reassign `mod1.var`, `mod2.var` still points to the same thing.

What you want to do is just:

```
import mod2
```

Then access and assign the variable as `mod2.var`.

It's important to note that global variables in Python are not truly global. They are global only to the module they're declared in. To access global variables inside another module, you use the `module.variable` syntax. The `global` statement can be used inside a function to allow a module-global name to be assigned to (without it, assigning to a variable makes it a local variable in that function). It has no other effect.

Understanding global variable in Python

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