How does python startswith work?
I am not able to understand the behavior of the str.startswith
method.
If I execute "hello".startswith("")
it returns True. Ideally it doesn't starts with empty string.
>>> "hello".startswith("")
True
The documentation states:
Return
True
if string starts with the prefix, otherwise returnFalse
.prefix
can also be a tuple of prefixes to look for.
So how does the function work?
Solution 1:
str.startswith()
can be expressed in Python code as:
def startswith(source, prefix):
return source[:len(prefix)] == prefix
It tests if the first len(prefix)
characters of the source string are equal to the prefix. If you pass in a prefix of length zero, that means the first 0 characters are tested. A string of length 0 is always equal to any other string of length 0.
Note that this applies to other string tests too:
>>> s = 'foobar'
>>> '' in s
True
>>> s.endswith('')
True
>>> s.find('')
0
>>> s.index('')
0
>>> s.count('')
7
>>> s.replace('', ' -> ')
' -> f -> o -> o -> b -> a -> r -> '
Those last two demos, counting the empty string or replacing the empty string with something else, shows that you can find an empty string at every position in the input string.
Solution 2:
A string p
is a prefix of a string s
if s = p + x
, so the empty string is a prefix of all strings (it's like 0, s = 0 + s
).