Python functions are almost like mathematical functions - you give the function some input (give the number 2 to the sqr function), it performs some computation and gives you an output (4, in the case of the sqr function). In fact, the only reason we are using a function is because we are interested in getting its output.
But this need not be the case always. In this chapter, we examine how to write functions which have side effects.
Most useful computer programs are closely connected with the real world. When you go to a bank to deposit or withdraw money, you are interacting with a computer program which knows everything about the accounting principles followed by that bank. Here is a toy Python program which tries to do a bit of banking!
>>>balance = 1000
>>>def deposit(amount):
... global balance
... balance = balance + amount
...
>>>
>>>def withdraw(amount):
... global balance
... balance = balance - amount
...
>>>
We first create a variable called balance initialized with the value 1000 (say we have 1000 Rupees balance initially). Then we define two simple functions deposit and withdraw. The interesting things about these functions are:
- Both functions do not use return
- There is a new keyword in the body of both functions - global
The line:
global balance
simply tells Python that the variable balance being used in the next line (and all the other lines of the function, if any) refers to the variable balance declared outside the function (such variables are called global variables in programming language terminology). Let’s see what happens when we call this function:
>>> balance
1000
>>> deposit(100)
>>> balance
1100
>>> deposit(30)
>>> balance
1130
>>>
Initially, the variable balance has value 1000. When you compare the call:
>>> deposit(100)
>>>
with the call:
>>> sqr(2)
4
>>>
one thing becomes clear - deposit is not returning any value, unlike sqr which gives back 4. This is because we simply have not used a return statement in the body of deposit. Still, deposit does something useful; it adds 100 to the global variable balance. This is evident when we ask Python to display the value of balance - indeed, it has changed to 1100. Yet another call to deposit with argument 30 results in 30 being added to the balance.
Similar is the case when you withdraw some money from our toy Python bank:
>>> balance
1130
>>> withdraw(20)
1110
>>> withdraw(10)
1100
>>>
The two functions above (deposit and withdraw) are said to have a side effect - in this case, the side effect is altering the value of a global variable. Both these functions are useful precisely because they have this side effect.
If you go deeper into programming, you will see many situations where you will define functions mostly to generate some kind of side effect, rather than to evaluate some mathematical expressions. Those topics are beyond the scope of this book.