python – Using any() and all() to check if a list contains one set of values or another

Generally speaking:

all and any are functions that take some iterable and return True, if

in the case of all(), no values in the iterable are falsy;
in the case of any(), at least one value is truthy.

A value x is falsy iff bool(x) == False.
A value x is truthy iff bool(x) == True.

Any non-booleans in the iterable will be fine — bool(x) will map (or coerce, if you prefer) any x according to these rules: 0, 0.0, None, [], (), [], set(), and other empty collections get mapped to False, anything else to True. The docstring for bool uses the terms true/false for truthy/falsy, and True/False for the concrete boolean values.

In your specific code samples:

You misunderstood a little bit how these functions work. Hence, the following does something completely not what you thought:

if any(foobars) == big_foobar:

…because any(foobars) would first be evaluated to either True or False, and then that boolean value would be compared to big_foobar, which generally always gives you False (unless big_foobar coincidentally happened to be the same boolean value).

Note: the iterable can be a list, but it can also be a generator/generator expression (≈ lazily evaluated/generated list) or any other iterator.

What you want instead is:

if any(x == big_foobar for x in foobars):

which basically first constructs an iterable that yields a sequence of booleans—for each item in foobars, it compares the item to big_foobar and emits the resulting boolean into the resulting sequence:

tmp = (x == big_foobar for x in foobars)

then any walks over all items in tmp and returns True as soon as it finds the first truthy element. Its as if you did the following:

In [1]: foobars = [big, small, medium, nice, ugly]                                        

In [2]: big_foobar = big                                                                          

In [3]: any([big == big_foobar, small == big_foobar, medium == big_foobar, nice == big_foobar, ugly == big_foobar])        
Out[3]: True

Note: As DSM pointed out, any(x == y for x in xs) is equivalent to y in xs but the latter is more readable, quicker to write and runs faster.

Some examples:

In [1]: any(x > 5 for x in range(4))
Out[1]: False

In [2]: all(isinstance(x, int) for x in range(10))
Out[2]: True

In [3]: any(x == Erik for x in [Erik, John, Jane, Jim])
Out[3]: True

In [4]: all([True, True, True, False, True])
Out[4]: False

python – Using any() and all() to check if a list contains one set of values or another

python – Using any() and all() to check if a list contains one set of values or another

Leave a Reply Cancel reply