Python password generator
To build on what else has been posted, validating your inputs is necessary and you have a lot of functions and loops that can be removed. 

String module

First, I would like to introduce you to the string module that is part of the python standard library. This module has string constants that you can import and use:

&gt;&gt;&gt; import string
&gt;&gt;&gt; string.ascii_lowercase
'abcdefghijklmnopqrstuvwxyz'
&gt;&gt;&gt; string.ascii_uppercase
'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
&gt;&gt;&gt; string.ascii_letters
'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ'
&gt;&gt;&gt; string.punctuation
'!"#$%&amp;\'()*+,-./:;&lt;=&gt;?@[\\]^_`{|}~'


Using these would allow you to get rid of your first three functions (letter, upper_case, and special_char). Similarly to what Joe said, it would be better to make your character space a variable to avoid creating extra objects, and remove the all_char function at the same time.

create_password function

This function is clear and works okay, but can also be improved. As you are doing the same operation in each iteration of the loop (random.choice) this is and ideal scenario to use a list comprehension.

# Before
def create_password(password, n):
    for x in range(n):
        password += random.choice(all_char())
    return password

# After
def create_password(n):
    return ''.join([random.choice(all_char()) for _ in range(n)])


This works by creating a list where a random character is selected from all_char() and then joining them all together. The underscore is a 'throwaway' variable here, see here.

Alternatively, as we are already using the random module we could use random.choices (but only in python3.6 and above) as this will return a list of k size:

def create_password(n):
    return ''.join(random.choices(all_char(), k=n))


check_password function

In general, simple statements are better than complex statements. Therefore, I would change the first line of this function to be:

nletter, nupper, nspecial = 0, 0, 0


As this is clearer.

The for loop works, and could be improved by using the string module constants, though they would need to be made into lists.

for x in password:
    if x in list(string.ascii_lowercase):
        nletter += 1
    if x in list(string.ascii_uppercase):
        nupper += 1
    if x in list(string.punctuation):
        nspecial += 1


Another way to test if two strings overlap is to use sets; but this is getting more complex.



Altogether

import random
import string

def create_password(n, chars):
    return ''.join(random.choices(chars, k=n)) 

def check_password(password):
    nletter, nupper, nspecial = 0, 0, 0
    for x in password:
        if x in list(string.ascii_lowercase):
            nletter += 1
        if x in list(string.ascii_uppercase):
            nupper += 1
        if x in list(string.punctuation):
            nspecial += 1
    if nletter &gt; 0 and nupper &gt; 0 and nspecial &gt; 0:
        return True
    else:
        return False

def main():
    characters = string.ascii_letters + string.punctuation
    n = 0
    while n &lt; 5:
        try:
            n = int(input('Enter your password length (minimum 5): '))
        except ValueError:
            print('Not a number')

    password = ''
    while not check_password(password):
        password = create_password(n, characters)

    print('Generated password : {}'.format(password))

run = True
while run:
    try:
        main()
    except KeyboardInterrupt:
        run = False
        print('Ctrl+c caught exiting')