Understanding pythons lstrip method on strings

Question

I ran into what I believe is a bug, and I'm searching for affirmation or that I am not arrangement how this method functions. 

Here's my essential output:

(Pdb) x = 'KEY_K'

(Pdb) x.lstrip('K')

'EY_K'

(Pdb) x.lstrip('KE')

'Y_K'

(Pdb) x.lstrip('KEY')

'_K'

(Pdb) x.lstrip('KEY_')

''

(Pdb) import sys

(Pdb) sys.version

'2.7.11 (default, Dec  5 2015, 14:44:47) \n[GCC 4.2.1 Compatible Apple LLVM 7.0.0 (clang-700.1.76)]

My understaning is that the last 'lstrip' in that model ought to have returned 'K', however it didn't. Does anybody know why?

Answer 1

From the docs: 

lstrip(...) S.lstrip([chars]) -> string or unicode

Return a copy of the string S with leading whitespace removed. If chars is given and not None, remove characters in chars instead. If chars is unicode, S will be converted to unicode before stripping

'K' is in 'KEY_', that is the reason your last model returns ''. 

Note that 'K' would not have been taken out whenever gone before by a character that isn't in 'KEY_':

>>> 'KEY_xK'.lstrip('KEY_')

'xK'

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