I’m looking for a string.contains or string.indexof method in Python.

I want to do:

if not somestring.contains("blah"):
   continue

You can use the in operator:

if "blah" not in somestring: 
    continue

If it’s just a substring search you can use string.find("substring").

You do have to be a little careful with find, index, and in though, as they are substring searches. In other words, this:

s = "This be a string"
if s.find("is") == -1:
    print("No 'is' here!")
else:
    print("Found 'is' in the string.")

It would print Found 'is' in the string. Similarly, if "is" in s: would evaluate to True. This may or may not be what you want.

Does Python have a string contains substring method?

Yes, but Python has a comparison operator that you should use instead, because the language intends its usage, and other programmers will expect you to use it. That keyword is in, which is used as a comparison operator:

>>> 'foo' in '**foo**'
True

The opposite (complement), which the original question asks for, is not in:

>>> 'foo' not in '**foo**' # returns False
False

This is semantically the same as not 'foo' in '**foo**' but it’s much more readable and explicitly provided for in the language as a readability improvement.

Avoid using __contains__, find, and index

As promised, here’s the contains method:

str.__contains__('**foo**', 'foo')

returns True. You could also call this function from the instance of the superstring:

'**foo**'.__contains__('foo')

But don’t. Methods that start with underscores are considered semantically private. The only reason to use this is when extending the in and not in functionality (e.g. if subclassing str):

class NoisyString(str):
    def __contains__(self, other):
        print('testing if "{0}" in "{1}"'.format(other, self))
        return super(NoisyString, self).__contains__(other)

ns = NoisyString('a string with a substring inside')

and now:

>>> 'substring' in ns
testing if "substring" in "a string with a substring inside"
True

Also, avoid the following string methods:

>>> '**foo**'.index('foo')
2
>>> '**foo**'.find('foo')
2

>>> '**oo**'.find('foo')
-1
>>> '**oo**'.index('foo')

Traceback (most recent call last):
  File "<pyshell#40>", line 1, in <module>
    '**oo**'.index('foo')
ValueError: substring not found

Other languages may have no methods to directly test for substrings, and so you would have to use these types of methods, but with Python, it is much more efficient to use the in comparison operator.

Performance comparisons

We can compare various ways of accomplishing the same goal.

import timeit

def in_(s, other):
    return other in s

def contains(s, other):
    return s.__contains__(other)

def find(s, other):
    return s.find(other) != -1

def index(s, other):
    try:
        s.index(other)
    except ValueError:
        return False
    else:
        return True



perf_dict = {
'in:True': min(timeit.repeat(lambda: in_('superstring', 'str'))),
'in:False': min(timeit.repeat(lambda: in_('superstring', 'not'))),
'__contains__:True': min(timeit.repeat(lambda: contains('superstring', 'str'))),
'__contains__:False': min(timeit.repeat(lambda: contains('superstring', 'not'))),
'find:True': min(timeit.repeat(lambda: find('superstring', 'str'))),
'find:False': min(timeit.repeat(lambda: find('superstring', 'not'))),
'index:True': min(timeit.repeat(lambda: index('superstring', 'str'))),
'index:False': min(timeit.repeat(lambda: index('superstring', 'not'))),
}

And now we see that using in is much faster than the others. Less time to do an equivalent operation is better:

>>> perf_dict
{'in:True': 0.16450627865128808,
 'in:False': 0.1609668098178645,
 '__contains__:True': 0.24355481654697542,
 '__contains__:False': 0.24382793854783813,
 'find:True': 0.3067379407923454,
 'find:False': 0.29860888058124146,
 'index:True': 0.29647137792585454,
 'index:False': 0.5502287584545229}

if needle in haystack: is the normal use, as @Michael says — it relies on the in operator, more readable and faster than a method call.

If you truly need a method instead of an operator (e.g. to do some weird key= for a very peculiar sort…?), that would be 'haystack'.__contains__. But since your example is for use in an if, I guess you don’t really mean what you say;-). It’s not good form (nor readable, nor efficient) to use special methods directly — they’re meant to be used, instead, through the operators and builtins that delegate to them.

in Python strings and lists

Here are a few useful examples that speak for themselves concerning the in method:

"foo" in "foobar"
True

"foo" in "Foobar"
False

"foo" in "Foobar".lower()
True

"foo".capitalize() in "Foobar"
True

"foo" in ["bar", "foo", "foobar"]
True

"foo" in ["fo", "o", "foobar"]
False

["foo" in a for a in ["fo", "o", "foobar"]]
[False, False, True]

Caveat. Lists are iterables, and the in method acts on iterables, not just strings.

If you are happy with "blah" in somestring but want it to be a function/method call, you can probably do this

import operator

if not operator.contains(somestring, "blah"):
    continue

All operators in Python can be more or less found in the operator module including in.

So apparently there is nothing similar for vector-wise comparison. An obvious Python way to do so would be:

names = ['bob', 'john', 'mike']
any(st in 'bob and john' for st in names) 
>> True

any(st in 'mary and jane' for st in names) 
>> False

You can use y.count().

It will return the integer value of the number of times a sub string appears in a string.

For example:

string.count("bah") >> 0
string.count("Hello") >> 1

Here is your answer:

if "insert_char_or_string_here" in "insert_string_to_search_here":
    #DOSTUFF

For checking if it is false:

if not "insert_char_or_string_here" in "insert_string_to_search_here":
    #DOSTUFF

OR:

if "insert_char_or_string_here" not in "insert_string_to_search_here":
    #DOSTUFF

You can use regular expressions to get the occurrences:

>>> import re
>>> print(re.findall(r'( |t)', to_search_in)) # searches for t or space
['t', ' ', 't', ' ', ' ']