Python | assert keyword

Last Updated : 25 Nov, 2018

Assertions in any programming language are the debugging tools which help in smooth flow of code. Assertions are mainly assumptions that a programmer knows always wants to be true and hence puts them in code so that failure of them doesn’t allow the code to execute further.

In python assert keyword helps in achieving this task. This statement simply takes input a boolean condition, which when returns true doesn’t return anything, but if it is computed to be false, then it raises an AssertionError along with the optional message provided.

Syntax : assert condition, error_message(optional)

Parameters :
condition : The boolean condition returning true or false.
error_message : The optional argument to be printed in console in case of AssertionError

Returns :
Returns AssertionError, in case the condition evaluates to false along with the error message which when provided.

# Python 3 code to demonstrate  
# working of assert  
  
# initializing number 
a = 4
b = 0
  
# using assert to check for 0 
print ("The value of a / b is : ") 
assert b != 0, "Divide by 0 error"
print (a / b) 

Output :

The value of a/b is :

Runtime Exception :

Traceback (most recent call last):
  File "/home/40545678b342ce3b70beb1224bed345f.py", line 10, in 
    assert b != 0, "Divide by 0 error"
AssertionError: Divide by 0 error

Practical Application : This has a much greater utility in testing and Quality assurance role in any development domain. Different types of assertions are used depending upon the application. Below is the simpler demonstration of program that only allows only the batch with all hot food to be dispatched, else rejects whole batch.

# Python 3 code to demonstrate  
# working of assert  
# Application 
  
# initializing list of foods temperatures 
batch = [ 40, 26, 39, 30, 25, 21] 
  
# initializing cut temperature 
cut = 26
  
# using assert to check for temperature greater than cut 
for i in batch: 
    assert i >= 26, "Batch is Rejected"
    print (str(i) + " is O.K" ) 

Output :

40 is O.K
26 is O.K
39 is O.K
30 is O.K

Runtime Exception :

Traceback (most recent call last):
  File "/home/bd45fb65343814a85b6c19bbe366b419.py", line 13, in 
    assert i >= 26, "Batch is Rejected"
AssertionError: Batch is Rejected

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