Not x.islower() Versus x.isupper Output Results

[Christopher Reimer]

Greetings,

I was playing around with a piece of code to remove lowercase letters 
and leave behind uppercase letters from a string when I got unexpected 
results.

     string = 'Whiskey Tango Foxtrot'

     list(filter((lambda x: not x.islower()), string))

     ['W', ' ', 'T', ' ', 'F']

Note the space characters in the list.

     list(filter((lambda x: x.isupper()), string))

     ['W', 'T', 'F']

Note that there are no space characters in the list.

Shouldn't the results between 'not x.islower()' and 'x.isupper()' be 
identical?

The final form of this code is this:

     list(filter(str.isupper, string))

     ['W', 'T', 'F']

Thank you,

Chris R.