Python assert statement: How to set the error message in the AssertionError

Python assert checks a condition and raises the AssertionError if it is false. No error happens in the following code.

assert True

Python raises the AssertionError if the statement is false.

assert False

# AssertionError

Simple Examples

x = 3

assert x == 2

# Traceback (most recent call last):
#   File "/Users/serif/python/test/util/tuple.py", line 3, in <module>
#     assert x == 2
# AssertionError

Python raises the AssertionError if the condition is false.

a = 1
b = 2

assert a + 1 == b

Error message

a = 1
b = 2

assert a + 2 == b, 'Not Equal'

# AssertionError: Not Equal

You can set an error message that is printed only when a condition is false. There needs a comma between the condition and message.