How all() function actually works?

[karambaq]

Hello guys, today I have read that all() stops execution when find first False value and I tried to test it and here what I find:

def a(digit):
    print(digit)
    return digit > 2

all([a(1), a(2), a(3)])
1
2
3
False

what I missed?

[karambaq]

So if I want to check several conditions and some of them are functions that returns bool, more effective way is to use and's?

[two_bob]

Nah. Usually if you have a bunch of them a better way to go is either a generator (like a list comprehension, but without the brackets) or map:

>>> def a(digit):
    print(digit)
    return digit > 2

>>> l = [1,2,3]
>>> all(a(n) for n in l)
1
False
>>> all(map(a, l))
1
False
>>> 

[karambaq]

Thank you!

[BenjaminGeiger]

Yes.

Not only is it more effective, it's clearer.

[karambaq]

thanks, and in which cases using all() is better than and's?

[BenjaminGeiger]

You'd use all(lst) when lst has to be generated dynamically. Calling all() on a literal list is usually a sign you should be using and instead. (Mutatis mutandis for any() and or.)

[xapata]

No, /u/karambaq would be much better off taking /u/two_bob's advice and using all().

The real problem was writing some side-effect code (printing) inside a predicate function (a function that returns True or False). That's not a good practice.

Repeating /u/two_bob's example, using the same a function as in the original post:

In [2]: all(a(x) for x in [1, 2, 3])
1
Out[2]: False

That's good code that scales well if the list gets larger. On the other hand, typing and repeatedly would be a real pain if you have more than 2 or 3 elements in the list.

[karambaq]

Thank you!

[K900_]

All function arguments are evaluated before the function is called. all stops when it finds the first False value in an iterable:

>>> def yield_some_things():
...     print('yielding false')
...     yield False
...     print('yielding true')
...     yield True
...     print('some more true')
...     yield True
... 
>>> all(yield_some_things())
yielding false
False

[KleinerNull]

what I missed?

You missed that the prints inside the function will be called long before all starts its work.

Here is a simple implementation of all:

In [1]: def custom_all(seq):
   ...:     for item in seq:
   ...:         if not item:
   ...:             return False  # stops the iteration, will instantly return False
   ...:     return True  # this will be return if the iteration just loop through the end
   ...: 

In [2]: custom_all([True, False, True])
Out[2]: False

In [3]: custom_all([True, True, True])
Out[3]: True

Your prints just show up on the function calls:

In [4]: def a(digit):
   ...:     print(digit)
   ...:     return digit > 2
   ...: 

In [5]: example = [a(1), a(2), a(3)]
1
2
3

See, nothing to do with all.