How would I check a string for a certain letter in Python?

Use the in keyword without is.

if x in dog:
    print Yes!

If youd like to check for the non-existence of a character, use not in:

if x not in dog:
    print No!

in keyword allows you to loop over a collection and check if there is a member in the collection that is equal to the element.

In this case string is nothing but a list of characters:

dog = xdasds
if x in dog:
     print Yes!

You can check a substring too:

>>> x in xdasds
True
>>> xd in xdasds
True
>>> 
>>> 
>>> xa in xdasds
False

Think collection:

>>> x in [x, d, a, s, d, s]
True
>>>

You can also test the set membership over user defined classes.

For user-defined classes which define the __contains__ method, x in y is true if and only if y.__contains__(x) is true.

If you want a version that raises an error:

string to search.index(needle)

If you want a version that returns -1:

string to search.find(needle)

This is more efficient than the in syntax