# Testing in Python using doctest module

PythonServer Side ProgrammingProgramming

We know docstring gives extra information about the function and classes in Python. We can also use it for testing of the functions using the doctest module. The doctest modules execute the code which starts with >>> and compares it against the expected output.

Follow the below steps to write a function with doctest.

• Import the doctest module.

• Write the function with docstring. Inside the docstring, write the following two lines for testing of the same function.

• >>>function_name(*args).

• Expected output.

• Write the function code.

• Now, call the doctest.testmod(name=function_name, verbose=True) function for testing. We can't see the test results if the verbose set to False and all tests are passed. It's better to set it as True.

## Example

Let's write a simple function with doctest.

# importing the module
import doctest
# function
def numbers_sum(*args) -> int:
"""
This function returns the sum of all the argumets
Shell commands for testing
incoking the function followed by expected output:
>>> numbers_sum(1, 2, 3, 4, 5)
15
>>> numbers_sum(6, 7, 8)
21
"""
return sum(args)
# invoking the testmod function
doctest.testmod(name='numbers_sum', verbose=True)

If you run the above code, you will get the following results.

Trying:
numbers_sum(1, 2, 3, 4, 5)
Expecting:
15
ok
Trying:
numbers_sum(6, 7, 8)
Expecting:
21
ok
numbers_sum
1 items passed all tests:
2 tests in numbers_sum.numbers_sum
2 tests in 2 items.
2 passed and 0 failed.
Test passed.
TestResults(failed=0, attempted=2)



If you see the output, there is a word ok after each test. That means expected output and actual output are matched. You can check the test results at the end of the output.

## Example

Let's see what happens when tests are failed. Run the same example with wrong outputs.

# importing the module
import doctest
# function
def numbers_sum(*args) -> int:
"""
This function returns the sum of all the argumets
Shell commands for testing
incoking the function followed by expected output:
>>> numbers_sum(1, 2, 3, 4, 5)
10
>>> numbers_sum(6, 7, 8) 23
""" return sum(args)
# invoking the testmod function
doctest.testmod(name='numbers_sum', verbose=True)

## Output

If you execute the above program, you will get the following results.

Trying:
numbers_sum(1, 2, 3, 4, 5)
Expecting:
10
**********************************************************************
File "__main__", line 10, in numbers_sum.numbers_sum
Failed example:
numbers_sum(1, 2, 3, 4, 5)
Expected:
10
Got:
15
Trying:
numbers_sum(6, 7, 8)
Expecting:
23
**********************************************************************
File "__main__", line 12, in numbers_sum.numbers_sum
Failed example:
numbers_sum(6, 7, 8)
Expected:
23
Got:
21
numbers_sum
**********************************************************************
2 of 2 in numbers_sum.numbers_sum
2 tests in 2 items.
0 passed and 2 failed.
***Test Failed*** 2 failures.
TestResults(failed=2, attempted=2)

If you see the test results, 2 are failed. You can also check the expected and actual output in the output.

## Conclusion

If you have any doubts in the tutorial, mention them in the comment section.

Published on 12-Feb-2020 12:56:29