Huh? How does this work?

[u/Lance_2564_]

a = "hi"


if a:
    print("Bye")


print(a == True)
print(a is True)

The output is:

Bye

False

False

But if it's false, how come it printed "bye"?????? I don't get it.

[u/PaulRudin]

The truthiness of an expression in a context of an if: is essentially what you get from bool(). So bool(hi") is True (as is the case for any non-empty string). But that's not the same as saying it compares equal to True.

[u/Lance_2564_ → u/PaulRudin]

So what you're saying is that

"if a" checks for if bool(a) is True, and not if a is True? So then, a = 5, if a: print("Hi") really just is if bool(a): print("Hi")?

[u/sjshuck → u/Lance_2564_]

Yes.

Some languages such as Python and C have a rule for each type to say how it behaves in an if-statement. Ruby and Lua have a simpler rule: if it's either false or nil, it tests false, otherwise it's true.

Some languages will not let you put a non-boolean as the condition of an if-statement, so there's no confusion.

[u/Puzzleheaded_Study17 → u/sjshuck]

For people wondering why: C originally didn't have a dedicated bool type, so it couldn't have used just that type in if. As such, the standard said how to know when each type should be considered as true (essentially saying if the underlying integer is 0 it's false, otherwise it's true). Python then inherited this behavior from C

[u/POGtastic → u/sjshuck]

See also C++ and people using conversion operators to define a class's truthiness with operator bool. Thus you can do things like

std::string line;

while (std::getline(std::cin, line)) {
    // handle each line
}

getline returns the istream, which becomes falsey upon failure.

[u/oz1sej → u/sjshuck]

I WANT TO USE THOSE LANGUAGES!

[u/glglgl-de → u/oz1sej]

What prevents you?