title: Numpy array declaration error
body: <p>I have written the following code to extract features from an image. The feature vector once extracted from the featex function, needs to be appended to a large feature 2D array containing the features of all images used for training. The code is as follows:</p>
  
  <pre><code>for dirs, path, files in os.walk("wallet_training/"):
      for filename in files:
              f=os.path.join("wallet_training",filename)
              I=Image.open("wallet_training/1(1).jpeg")
              I=imresize(I,(256,256))
              p=featex(I)
              features=np.vstack([features],[p])
  
  print features.shape
  </code></pre>
  
  <p>It gives the following error:</p>
  
  <pre><code>NameError: name 'features' is not defined
  </code></pre>
  
  <p>Can someone help me why this error is coming, because as far as i remember variables in python dont need to be defined beforehand.</p>
  
  <p>Thank you in advance.</p>