By Django, you can use get_random_string function in django.utils.crypto module.

get_random_string(length=12,
    allowed_chars=u'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789')
    Returns a securely generated random string.

    The default length of 12 with the a-z, A-Z, 0-9 character set returns
    a 71-bit value. log_2((26+26+10)^12) =~ 71 bits

Example:

get_random_string()
u'ngccjtxvvmr9'

get_random_string(4, allowed_chars='bqDE56')
u'DDD6'

But if you don't want to have Django, here is independent code of it:

Code:

import random
import hashlib
import time

SECRET_KEY = 'PUT A RANDOM KEY WITH 50 CHARACTERS LENGTH HERE !!'

try:
    random = random.SystemRandom()
    using_sysrandom = True
except NotImplementedError:
    import warnings
    warnings.warn('A secure pseudo-random number generator is not available '
                  'on your system. Falling back to Mersenne Twister.')
    using_sysrandom = False


def get_random_string(length=12,
                      allowed_chars='abcdefghijklmnopqrstuvwxyz'
                                    'ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789'):
    """
    Returns a securely generated random string.

    The default length of 12 with the a-z, A-Z, 0-9 character set returns
    a 71-bit value. log_2((26+26+10)^12) =~ 71 bits
    """
    if not using_sysrandom:
        # This is ugly, and a hack, but it makes things better than
        # the alternative of predictability. This re-seeds the PRNG
        # using a value that is hard for an attacker to predict, every
        # time a random string is required. This may change the
        # properties of the chosen random sequence slightly, but this
        # is better than absolute predictability.
        random.seed(
            hashlib.sha256(
                ("%s%s%s" % (
                    random.getstate(),
                    time.time(),
                    SECRET_KEY)).encode('utf-8')
            ).digest())
    return ''.join(random.choice(allowed_chars) for i in range(length))

If you want no repetitions:

import string, random
''.join(random.sample(string.ascii_lowercase, X))

If you DO want (potential) repetitions:

import string, random
''.join(random.choice(string.ascii_lowercase) for _ in xrange(X)))

That's assuming that by a-z you mean "ASCII lowercase characters", otherwise your alphabet might be expressed differently in these expression (e.g., string.lowercase for "locale dependent lowercase letters" that may include accented or otherwise decorated lowercase letters depending on your current locale).

Variable string, variable length:

>>>import random, string    
>>>max_rand_str_length = 10
>>> def genCustomString():
...    return ''.join(random.choice(string.lowercase) for _ in range(random.choice(range(1, max_rand_str_length))))
... 
>>> genCustomString()
'v'
>>> genCustomString()
'mkmylzc'
>>> genCustomString()
'azp'
>>> genCustomString()