title: Python function os.listdir doesnt show all files in directory
body: <p>Hi I'm trying load couple of files  with <code>os.listdir</code> but it skips several files. </p>
  
  <p>Format of files in directiy is:</p>
  
  <pre><code>a_0.csv
  a_1.csv
  a_2.csv
    ·
    ·
    ·
  a_25.csv
  </code></pre>
  
  <p>But the <code>os.listdir</code> only take file <em>0,1,10,11..</em> 
  File with numbers <em>3,4,5...</em> aren't in in output object. </p>
  
  <p>And here is my function:</p>
  
  <pre><code>def best_fit_from_attemp(self):
      arr =[]
      for i in os.listdir(self.path_to_files):
          print("i",i)
          if i[0] == 'f' and  i[1] == 'i' and i[2] == 't':
              print("searching for smallest number... ",i)
              arr.append(self.find_min_fitness(i))
  </code></pre>
  
  <p>Output:</p>
  
  <pre class="lang-none prettyprint-override"><code>i fitness_gen_0.csv
  searching for smallest number...  fitness_gen_0.csv
  i fitness_gen_1.csv
  searching for smallest number...  fitness_gen_1.csv
  i fitness_gen_10.csv
  searching for smallest number...  fitness_gen_10.csv
  i fitness_gen_11.csv
  searching for smallest number...  fitness_gen_11.csv
  </code></pre>