Understand Python function variable binding

Note from edx.org’s MITx 6.00.1x course – lecture 4 functions.

In Python function, each function call creates a new environment, which scopes bindings of formal parameters and values and of local variables (those created with assignments within body)

Scoping often called static or lexical because scope within which variable has value is defined by extent of code boundaries.

The sample codes are

def f(x):

       y=1

       x=x+y

       return x

x = 3

y = 2

z = f(x)

what is the variable binding procedure and what is the output?

these are the initial variables

f is a function

x is 3

y is 2

When z=f(x) is called , python create an local variable stack as below

x is passed by the paramters. y is the local variable which is assigned in the first line and it doesn’t have any relationship with the global y. see below:

After the z=f(x) finishes, the stack and variables is looking like that

Function local variable x is now set to 4, but the global variable is still 4, and you get the z as 4.

So at the end of the code, x = 3 , y = 2

Let’s do another test

def f(x):

       x=x+t

       t = t+ 1

       return x

x=2

z = f(x)

when the first z = f(2) is called.

Traceback (most recent call last):

  File "<stdin>", line 1, in <module>

  File "<stdin>", line 2, in f

NameError: global name 't' is not defined

Obviously there is an error as t is not defined.

But if we assign t, you will get another error

Traceback (most recent call last):

  File "<stdin>", line 1, in <module>

  File "<stdin>", line 2, in f

UnboundLocalError: local variable 't' referenced before assignment

Interesting, python knows you have got a ‘t’ but can’t use it. 

Now if we assign t first then use the global variable in the function, let’s see how it goes.

t = 3
def f(x):
    x=x+t       
    t = t+ 1
    return x
x=2
z = f(x)

at the end of the code, you will get the variables as:

x=2, t=4 (self increase in the f function) and z=5 (3+2)

wow. Python can use the global variables in the function without a global declare. Amazing but some times it is a little confusing as you may think where we got a variable, isn't it