code: def uniform_string_column(
      n, word_length, alphabet_size, missingness=0.0
  ):
      """
      Return an SArray of strings constructed uniformly randomly from the first
      'num_letters' of the lower case alphabet.
  
      Parameters
      ----------
      n : int
          Number of entries in the output SArray.
  
      word_length : int
          Number of characters in each string.
  
      alphabet_size : int
          Number of characters in the alphabet.
  
      missingness : float, optional
          Probability that a given entry in the output is missing.
  
      Returns
      -------
      out : SArray
          One string "word" in each entry of the output SArray.
      """
      result = []
      letters = list(string.ascii_letters[:alphabet_size])
  
      for i in range(n):
          missing_flag = random.random()
          if missing_flag < missingness:
              result.append(None)
          else:
              word = []
              for j in range(word_length):
                  word.append(np.random.choice(letters))
              result.append("".join(word))
  
      return SArray(result)
  
imports: from __future__ import print_function
  from __future__ import division
  from __future__ import absolute_import
  import random
  import tempfile
  import shutil
  import math
  import string
  import numpy
  from pandas.util.testing import assert_frame_equal
  from ..None import SArray
  import turicreate
  import sys
  from six import StringIO
  import pytest
label_desc: Use image node_id 1 for calling a global function with example usage: uniform_string_column(n, word_length, alphabet_size, missingness) and returns: SArray