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Is there a way to check against all special characters?

Is there any way to test for all special characters in python other than manually putting them in, perhaps something similar to the .isalnum or .isalpha functions? I'm relatively new to coding, so I have no idea.

Assuming that any non alphanumeric character counts as special, you can add not in front isalnum and will return true when there's any special character:
test = "1$%a"
print not test.isalnum()
# returns true

You could define your own is_alphanumeric function:
from string import digits, letters
def is_alphanumeric(mystring):
""" Returns true if all characters of `mystring` are either letters or digits:
>>> is_alphanumeric('hello wörld')
True
>>> is alphanumeric('Hello World!')
False
"""
return all(character in chain(digits, letters) for character in mystring)
If you want to restrict it to ascii:
from string import digits, letters_ascii
def is_alphanumeric_ascii(mystring):
""" Returns true if all characters of `mystring` are either ascii letters or digits:
>>> is_alphanumeric('hello wörld')
False
>>> is alphanumeric('Hello World')
True
"""
return all(character in chain(digits, letters_ascii) for character in mystring)

Is there a way to see if a string contains any symbols in Python?

I don't mean specific characters, I just mean anything that isn't alphanumeric. I've tried asking if the string contains only alphabetic and numeric characters like so:
if userInput.isalpha() and userInput.isdigit() == False:
print ("Not valid, contains symbols or spaces")
but this doesn't work and and denies all passwords I put in.

Firstly, you can't be only alpha and only numeric, so your expression will always be false.
Secondly the methods isalpha() and isdigit() equate to True or False, so no need to use == False.
I would suggest using .isalnum().
If that doesn't satisfy you're requirements you should use a regex.
alnum looks for both: https://docs.python.org/2/library/stdtypes.html#str.isalnum and works in Python 3.x and 2.
Example:
>>> 'sometest'.isalnum()
True
>>> 'some test'.isalnum()
False
>>> 'sometest231'.isalnum()
True
>>> 'sometest%231'.isalnum()
False
>>> '231'.isalnum()
True

You have three problems:
if a and b == False is not the same is if a == False and b == False;
if b == False should be written if not b; and
You aren't using str.isalnum, which saves you from the problem anyway.
So it should be:
if not userInput.isalnum():

How to check if a given character is considered as 'special' by the Python regex engine?

Is there an easy way to verify that the given character has a special regex function?
Of course I can collect regex characters in a list like ['.', "[", "]", etc.] to check that, but I guess there is a more elegant way.

You could use re.escape. For example:
>>> re.escape("a") == "a"
True
>>> re.escape("[") == "["
False
The idea is that if a character is a special one, then re.escape returns the character with a backslash in front of it. Otherwise, it returns the character itself.

You can use re.escape within all function as following :
>>> def checker(st):
... return all(re.escape(i)==i for i in st)
...
>>> checker('aab]')
False
>>> checker('aab')
True
>>> checker('aa.b3')
False

Per the documentation, re.escape will (emphasis mine):
Return string with all non-alphanumerics backslashed; this is useful
if you want to match an arbitrary literal string that may have regular
expression metacharacters in it.
So it tells you whether a character could be a meaningful one, not whether it is. For example:
>>> re.escape('&') == '&'
False
This is useful for processing arbitrary strings, as it ensures that all control characters are escaped, but not for telling you which actually needed to be. The simplest approach, in my view, is the one dismissed in the question:
char in set(r'.^$*+?{}[]\| ')

Elegance lies in the eyes of the beholder, however (IMHO) this (below) is the most generic/"timeproof" way of checking if a character is considered to be special by the Python Regex engine -
def isFalsePositive(char):
m = re.match(char, 'a')
if m is not None and m.end() == 1:
return True
else:
return False
def isSpecial(char):
try:
m = re.match(char, char)
except:
return True
if m is not None and m.end() == 1:
if isFalsePositive(char):
return True
else:
return False
else:
return True
P.S. -
isFalsePositive() may be overkill to check the special case of '.' (dot). :-)

Assert string only contains letters

I have seen methods like isAlpha(), but it accepts spaces and punctuations, which I don't want. Any way to check that a string contains only big or upper case alphabet letters?
E.g. psudo:
"asdf".isLetters() -> true
"as df".isLetters() -> false
"as. df:".isLetters() -> false

>>> "asdf".isalpha()
True
>>> "as df".isalpha()
False
>>> "as. df:".isalpha()
False
According to the documentation for .isalpha() it does what it seems you're after:
Return true if all characters in the string are alphabetic and there is at least one character, false otherwise.
To check for uppercase, use my_str.isupper()

import re
if re.match(r"^[A-Za-z]*$", some_string):
print "yey"!

applying a filter on a string in python

I have a user typing in his username and I only want valid strings to pass through, meaning only characters in [a-zA-Z0-9]. I am pretty new to python and unsure of the syntax.
Here's an example of what I want in code, which is to check through the username and return false upon a illegal character.:
def _checkInput(input):
for char in input:
if !(char in [a-zA-Z0-9]):
return False
return True
Thanks!

There is a method in string called isalnum. It does what you are trying to achieve.
In [7]: 'ab123fd'.isalnum()
Out[7]: True
In [8]: 'ab123fd **'.isalnum()
Out[8]: False

You need isalnum:
>>> name = raw_input('Enter your name: ')
Enter your name: foo_bar
>>> name.isalnum()
False
>>> name = raw_input('Enter your name: ')
Enter your name: foobar
>>> name.isalnum()
True

Python strings have lots of useful methods for doing this sort of check, such as:
str.isalnum()
str.isalpha()
str.isdigit()
str.islower()
str.istitle()
str.isupper()
What you need is str.isalnum() which returns true if all characters in the string are alphanumeric and there is at least one character.
>>> 'hello1'.isalnum()
True
>>> 'hello 1'.isalnum()
False
>>> 'hello!'.isalnum()
False
>>> ''.isalnum()
False
As the example above shows, letters and numbers are considered alphanumeric, but spaces and punctuation marks are not.
Also note that contrary to what would be mathematically pure, the empty string is not considered alphanumeric. However in most cases this actually what you need and certainly what you need in your case, as a user name of length zero does not make much sense.

That's very close to being Python:
def _checkInput(input):
for c in input:
if not (c in string.ascii_letters or c in string.digits):
return False
return True
This can also be solved with regular expressions, but the above is perhaps clearer and less complex.

You can easily check input strings using regular expressions:
>>> import re
>>> s = getinput()
>>> if not re.match(r'^[a-zA-Z0-9]+$', s)
... print "bad input"
Use * instead of + if the empty string is valid input too.
Using isalnum, as suggested in other answers, is a nice approach too, but with regular expressions you can easily adjust your check in case the requirements for input get more complex.

One way to achieve this is to use the regular expression module of Python. It is a standard library.
import re
_pmatcher = re.compile(r'[0-9a-zA-Z]*$')
def _checkInput(input):
return _pmatcher.match(input)
The r in front of the string is not a typo, it is to treat the string as raw, which you may want rather than typing escape characters.
You can refer to this Python 2.7 Documents (or your chosen version of Python)

You may need in the future to verify that all the characters of a string are present in a particular list.
Without regex, it is possible like that:
ch = 'becdi30!&'
okchars = 'abcdefghijk012345,;:!&-'
print all(c in okchars for c in ch)
result
True