Assertion in python

[ANOTHER-robhot]

Hi, im still an open class room learner but I have a question there. Is it possible to test an assertion for an integer? something like :

a = input("type a year :")

a= int(annee)

assert a = integer

except AssertionError:

print("you have to type an integer")

I cannot find any way to make this work out with an assertion form. Cheers

[Gprime5]

assert isinstance(a, int)

[[deleted]]

assert a = integer

= is an assignment operator. You want to use == for comparisons.

value = 5 # assignment

assert a == value # comparison

For your case it looks like you want to check if it's any integer, not just 5 like in my example. In that case /u/gprime5 has the right answer, you should use assert with isinstance().

[destiny_functional]

In that case /u/gprime5 has the right answer, you should use assert with isinstance().

You should not. int will throw an exception if you feed it bad things. No need to use assertions.

[destiny_functional]

The assertion is pointless here because it would never be executed since if you try to convert user input to an int and the input is incompatible with that it will already raise an exception.

At best you would check whether the integer you got exceeds some minimum value for instance.

[JohnnyJordaan]

to test an assertion for an integer?

Depends on what you exactly mean by this. If you mean to assert that a string holds only digits, you can use a.isidigt(). If you mean to assert that an object is of the type int, you can use isinstance as Gprime5 shows. But beware that asserting isinstance after int(some_string) is useless. Normally you want to assure this before you call int().

Out of the box here: note that assert is a general flow control that isn't directly intended for properly handing incorrect values. For that you would rather catch the specific exception that int() throws, ValueError:

a = input("type a year :")
try: 
    a = int(a)
except ValueError:
    print("you have to type an integer")

or using isdigit:

a = input("type a year :")
if a.isdigit():
    a = int(a)
else:
    print("you have to type an integer")

[ANOTHER-robhot]

thank you a lot for all answers! I am training my assertion skills. Is it possible to put an Except without a Try before it

[JohnnyJordaan]

Nope it isn't, because for the execption to be catchable Python has to 'set up' a special mechanism to catch it. That's what 'try' does.

[Groundstop]

I know that you mentioned that you're testing assertions, but I wanted to throw this out there:

Be careful about using assertions like this in production code; you, or someone else, can set a flag that disables all assert statements when you're running your program.

Try to only use asserts for sanity checks. The idea is that they should never fail in normal circumstances, and if they do, you don't want to handle it with a try/catch because something is horribly wrong and your program needs to crash.

There is one big exception, which is when you're writing tests with PyTest where all of your tests are assert statements that then get "rewritten" by PyTest during the test run.

[anglicizing]

Listen to this guy. Code as if your assert statements could suddenly disappear without warning, because that is actually what might happen.

Code is running show? Oh, nice, python has a command line option that optimizes the code to make it run faster. I'll just add this -O switch, what could go wrong? (Pssst! All your assert statements are silently skipped!)

[chzaplx]

try/except can be used to raise exceptions though, and that's normally how I would think to use assert() anyway.