What is the use of "assert"?
The assert
statement exists in almost every programming language. It helps detect problems early in your program, where the cause is clear, rather than later when some other operation fails.
When you do...
assert condition
... you're telling the program to test that condition, and immediately trigger an error if the condition is false.
In Python, it's roughly equivalent to this:
if not condition:
raise AssertionError()
Try it in the Python shell:
>>> assert True # nothing happens
>>> assert False
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AssertionError
Assertions can include an optional message, and you can disable them when running the interpreter.
To print a message if the assertion fails:
assert False, "Oh no! This assertion failed!"
Do not use parenthesis to call assert
like a function. It is a statement. If you do assert(condition, message)
you'll be running the assert
with a (condition, message)
tuple as first parameter.
As for disabling them, when running python
in optimized mode, where __debug__
is False
, assert statements will be ignored. Just pass the -O
flag:
python -O script.py
See here for the relevant documentation.
Watch out for the parentheses. As has been pointed out above, in Python 3, assert
is still a statement, so by analogy with print(..)
, one may extrapolate the same to assert(..)
or raise(..)
but you shouldn't.
This is important because:
assert(2 + 2 == 5, "Houston we've got a problem")
won't work, unlike
assert 2 + 2 == 5, "Houston we've got a problem"
The reason the first one will not work is that bool( (False, "Houston we've got a problem") )
evaluates to True
.
In the statement assert(False)
, these are just redundant parentheses around False
, which evaluate to their contents. But with assert(False,)
the parentheses are now a tuple, and a non-empty tuple evaluates to True
in a boolean context.
As other answers have noted, assert
is similar to throwing an exception if a given condition isn't true. An important difference is that assert statements get ignored if you compile your code with the optimization option -O
. The documentation says that assert expression
can better be described as being equivalent to
if __debug__:
if not expression: raise AssertionError
This can be useful if you want to thoroughly test your code, then release an optimized version when you're happy that none of your assertion cases fail - when optimization is on, the __debug__
variable becomes False and the conditions will stop getting evaluated. This feature can also catch you out if you're relying on the asserts and don't realize they've disappeared.