When does Python evaluate a variable in a function definition

Question

I am confused as when does Python evaluates a variable. For example if I do:

p=4
def g():
    return p
p=5
print g()

I get 5, so in the definition of g, the variable p remains unevaluated. It is evaluated when we call g

However if I do:

def make_fun():
    p=5
    def f():
        return p
    return f

f=make_fun()
p=6
print f()

I get 5. Why does notf contains the unevaluated variable p? I would like to have a clear idea as of when precisely the evaluation of variables takes place.

Simeon Visser · Accepted Answer · 2013-12-30 18:19:16Z

6

Python executes the code as it loads the module / script. Hence in your first example it's not "evaluated when we call g", it simply means that the latest value of p is 5 at the time when g is executed. Python will lookup p and it'll return the current value (5).

In your second example p is a local variable. That means it's not affected by p = 6 in the global scope. Or to be clear, the p in f() in your second example is the local variable set within make_fun(). So that's the value you'll get back, it's not the p at the outer (global) scope, which is a different p.

answered Dec 30, 2013 at 18:19

Simeon Visser

123k19 gold badges192 silver badges185 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

LPH · Accepted Answer · 2013-12-30 18:18:50Z

3

p=4          #you are setting global p to 4
def g():
    return p
p=5          #you are setting global p to 5
print g()    #you are printing the result of g, which is not setting any value;

so 5 is printed; but here:

def make_fun():
    p=5          #you are setting local p to 5
    def f():
        return p
    return f

f=make_fun()
p=6              #you are setting global p to 6
print f()        #you printing the result of a function in f, which is getting the local p

so local p= 5 is printed;

hope i understood you right!

answered Dec 30, 2013 at 18:18

LPH

1,2959 silver badges16 bronze badges

Comments

Frank Riccobono · Accepted Answer · 2013-12-30 18:26:01Z

It's not a question of when evaluation takes place. In both of your examples, p is evaluated when the function is called. The real question is when does that variable get declared.

In your first example, p is declared outside of the function and therefore has global scope. Any changes to p impact it wherever it it referenced.

In your second example, you are declaring another variable p inside the function. This is local to the function and not impacted by any code outside of the function. Even if you then go ahead and declare another global p as you do in the line after f=make_fun(), this global variable is overridden within the function by the local variable.

As an experiment, try printing p after you print f() and you'll see that it's value will be 6 as you assigned it.

Collectives™ on Stack Overflow

When does Python evaluate a variable in a function definition

3 Answers 3

Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related