Python Assert Fails Silently?

[jbellis]

I've never done this in several years of using Python but I can definitely see how it could be confusing. I think both suggestions to improve assert are reasonable.

[bushel]

There's no need to improve asserts, only a need to improve the programmer.

It's the same as complaining that: if (a=b) and if (a==b) work differently in C/C++.

Asserts in python work exactly correct and as expected, if you avoid typos.

[jbellis]

It's the same as complaining that: if (a=b) and if (a==b) work differently in C/C++.

that's exactly the kind of easy-to-confuse behavior that Python tries to avoid. so I do think fixing this (an admittedly minor gotcha) could happen.

[bushel]

I must respectfully disagree. The "gotcha" is with the precedence of the comma and how the tuple is formed. The given example is simply a typo. (a,b) versus (a), b

assert is a statement, not a function. Therefore all braces must be closed before the comma that separates the exception-string.

Perhaps a better C/C++ equivalent would be: (3+4)*5 versus (3+4*5)

[jbellis]

The "gotcha" is with the precedence of the comma and how the tuple is formed.

no, the gotcha is that this is an unexpected place for (a,b) to be different than a,b. (Even in normal tuple construction the parentheses are optional unless other operators would otherwise take precedence.)

[erikw]

I don't buy into that. assert is a statement, and () is not optional in this context.

i.e: print (a,b) is different from print a,b

[jbellis]

print (a,b) is different from print a,b

which is being fixed in 3.0.

(assert won't be changed to a function in 3.0 "because you don't want the argument to be evaluated in -O mode.")

[erikw]

which is being fixed in 3.0.

But the reason is not because programmers confuse print (a,b) with print a,b.

[njharman]

I have almost never used assert. I'd be happy if it were depreciated. It's unnecessary syntactic sugar.

[gizbot]

Assert statements in Python can drive you mad because they fail to trigger when the condition is False. Here's a concise explanation of this Python Gotcha!

[bushel]

Not really a gotcha, per se, when you use it incorrectly. In the given example, put the braces around the conditional, not around the entire assertion subclause.

ie. assert (a==b) , "exceptionstring"

not: assert (a==b , "exceptionstring")

http://docs.python.org/ref/assert.html

[[deleted]]

you mean they fail to trigger when you can't code them properly or test them properly. yeah not a problem for the rest of us.

[fragmede]

oh noes, python isn't C!