How work counting True values in list of ints in Python?

Question

Could someone explain me why in for in l5 loop I got 2 True values, but from l5.count(True) I got only 1?? And why l5.count(not 0) returns only 1?

I know about other ways to get what I need: filter + lambda or sum + for + if, but I try understand it :)

Under part from Python console I've asked.

Python 3.9.0 (tags/v3.9.0:9cf6752, Oct 5 2020, 15:34:40) [MSC v.1927 64 bit (AMD64)] on win32 '''

l5 = [0, 0, 0, 1, 2]
l5.count(True)
Out[3]: 1
for x in l5:
    print(x, bool(x))
    
0 False
0 False
0 False
1 True
2 True
l5.count(0)
Out[5]: 3
l5.count(not 0)
Out[6]: 1
l5.count(not False)
Out[7]: 1

'''

count(True) will list only 1 value because there is only "1"(True) in the list. But bool(x) will return 1 for every non-zero value of x. That is why you have 2 true(s). — user6304394
– user6304394, Commented Mar 11, 2021 at 13:31
Just because you can call bool on the items in the list and the result is True or False doesn't mean the list actually contains these booleans. — MisterMiyagi
– MisterMiyagi, Commented Mar 11, 2021 at 13:44
It was not clear for me, but now I understand what happened with that '2' in my list. Thanks guys for Ur help. — matt91t
– matt91t, Commented Mar 11, 2021 at 20:15

Rishabh Kumar · Accepted Answer · 2021-03-11 13:49:16Z

False in python is numerically equal to 0 and True is numerically equal to 1.

and so not False will be equal to True which will be numerically equal to 1

l5 = [0, 0, 0, 1, 2]
l5.count(True) # is equivalent to l5.count(1)
Out[3]: 1
l5.count(not 0) # is equivalent to l5.count(1)
Out[6]: 1
l5.count(not False) # is equivalent to l5.count(1)
Out[7]: 1

Also

l5.count(False) # is equivalent to l5.count(0)
Out[8]: 3

When you do l5.count(True)

Internally python check it with each element in the list l5

0==True ?
0==True ?
0==True ?
1==True ?
2==True ?

Now as True is bool type and others are int, python does an implicit type conversion from bool to int (as to do a comparison type should be same)

So as True is numerically equivalent to 1:

Count=0

0==1 ? No
0==1 ? No
0==1 ? No
1==1 ? Yes; Count += 1 
2==1 ? No

Final output : 1

Now this was when type conversion from bool to int

In conversions from int to bool:

0 is treated as False and everything other than 0 , -1,-2,-3,... 1,2,3,.. are treated as True.

and so

>>> bool(-1)
True

>>> bool(1)
True

>>> bool(2)
True

>>> bool(0)
False

>>> not 2
False

# as 2 is equivalent to True and "not True" is False

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