Counting positive elements in a list with Python list comprehensions

Question

I have a list of integers and I need to count how many of them are > 0.
I'm currently doing it with a list comprehension that looks like this:

sum([1 for x in frequencies if x > 0])

It seems like a decent comprehension but I don't really like the "1"; it seems like a bit of a magic number. Is there a more Pythonish way to do this?

Answer 1

You could use len() on the filtered list:

len([x for x in frequencies if x > 0])

Answer 2

A slightly more Pythonic way would be to use a generator instead:

sum(1 for x in frequencies if x > 0)

This avoids generating the whole list before calling sum().

Answer 3

sum(x > 0 for x in frequencies)

bool is a subclass of int:

In [15]: isinstance(True,int)
Out[15]: True

and True's value is 1:

In [16]: True==1
Out[16]: True

Answer 4

How about this?

reduce(lambda x, y: x+1 if y > 0 else x, frequencies)

EDIT: With inspiration from the accepted answer from @~unutbu:

reduce(lambda x, y: x + (y > 0), frequencies)

Answer 5

This works, but adding bools as ints may be dangerous. Please take this code with a grain of salt (maintainability goes first):

sum(k>0 for k in x)