An **assertion** is a boolean expression. It is used to test a logical expression. An assertion is true if the logical expression that is being tested is true and there are no bugs in the program. Assertion testing can be used at any particular stage of the program. 

The illustration below describes an assertion test:

# Advantages

Assertion testing has the following advantages:

* Detects errors that are otherwise subtle and difficult to catch.
* Detect errors as soon as they occur.

# Disadvantages

Assertion testing has the following disadvantages:

* Does not report errors.
* Failed assertions can have side effects, and other bugs may not be caught.
* Can be time-consuming if there are multiple errors. 
* It cannot test all conditions. Some conditions can be tested conceptually, but not practically, through assertions.

# Example

The code snippet below shows an assertion test in Python:

* The `checkLimit` function tests that the `value` passed is greater than or equal to 0.

* The failed assertion in `Line 6` causes an error, so the subsequent code is not executed.

def checkLimit(value):
   assert (value >= 0),"Value cannot be less that 0"
   return True

print checkLimit(100) # Normal Case
print checkLimit(-10) # Error Case
print checkLimit(0) # Border Case


What is Assertion Testing?

Assertion testing uses boolean expressions to detect subtle errors early but may miss some conditions and have side effects if assertions fail.