Use and meaning of "in" in an if statement?

You are used to using the javascript if, and I assume you know how it works.

in is a Pythonic way of implementing iteration. It's supposed to be easier for non-programmatic thinkers to adopt, but that can sometimes make it harder for programmatic thinkers, ironically.

When you say if x in y, you are literally saying:

"if x is in y", which assumes that y has an index. In that if statement then, each object at each index in y is checked against the condition.

Similarly,

for x in y

iterates through x's in y, where y is that indexable set of items.

Think of the if situation this way (pseudo-code):

for i in next:
    if i == "0" || i == "1":
        how_much = int(next)

It takes care of the iteration over next for you.

Happy coding!

Since you claim to be used to JavaScript:

The Python in operator is similar to the JavaScript in operator.

Here's some JavaScript:

var d = {1: 2, 3: 4};
if (1 in d) {
    alert('true!');
}

And the equivalent Python:

d = {1: 2, 3: 4}
if 1 in d:
    print('true!')

With objects/dicts, they're nearly identical, both checking whether 1 is a key of the object/dict. The big difference, of course, is that JavaScript is sloppily-typed, so '1' in d would be just as true.

With arrays/lists, they're very different. A JS array is an object, and its indexes are the keys, so 1 in [3, 4, 5] will be true. A Python list is completely different from a dict, and its in operator checks the values, not the indexes, which tends to be more useful. And Python extends this behavior to all iterables.

With strings, they're even more different. A JS string isn't an object, so you will get a TypeError. But a Python str or unicode will check whether the other operand is a substring. (This means 1 in '123' is illegal, because 1 can't be a substring of anything, but '1' in '123' is true.)

With objects as objects, in JS there is of course no distinction, but in Python, objects are instances of classes, not dicts. So, in JS, 1 in d will be true for an object if it has a member or method named '1', but in Python, it's up to your class what it means—Python will call d.__contains__(1), then, if that fails, it tries to use your object as an utterable (by calling its __iter__, and, if that fails, by trying to index it with integers starting from 0).

Also, note that JS's in, because it's actually checking for object membership, does the usual JS method-resolution-order search, while Python's in, because it's checking for keys of a dict, members of a sequence, etc., does no such thing. So, technically, it's probably a bit closer to the hasOwnProperty method than the in operator.

It depends on what next is.

If it's a string (as in your example), then in checks for substrings.

>>> "in" in "indigo"
True
>>> "in" in "violet"
False
>>> "0" in "10"
True
>>> "1" in "10"
True

If it's a different kind of iterable (list, tuple, set, dictionary...), then in checks for membership.

>>> "in" in ["in", "out"]
True
>>> "in" in ["indigo", "violet"]
False

In a dictionary, membership is seen as "being one of the keys":

>>> "in" in {"in": "out"}
True
>>> "in" in {"out": "in"}
False

Using a in b is simply translates to b.__contains__(a), which should return if b includes a or not.

But, your example looks a little weird, it takes an input from user and assigns its integer value to how_much variable if the input contains "0" or "1".