AttributeError: 'numpy.ndarray' object has no attribute 'append'

I am trying to run the code presented on the second page:

http://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-00-introduction-to-computer-science-and-programming-fall-2008/video-lectures/lecture-20/lec20.pdf

At the bottom of the code you have to add these lines:

simFlips(100,100)

show()

Here is the error that I get when I run it on ubuntu:

Traceback (most recent call last):
  File "coin.py", line 36, in <module>
    simFlips(100,100)
  File "coin.py", line 16, in simFlips
    diffs.append(abs(heads - tails))
AttributeError: 'numpy.ndarray' object has no attribute 'append'

Please tell me what I'm doing wrong that gives me the last error. Thanks in advance!

Use numpy.concatenate(list1 , list2) or numpy.append() Look into the thread at Append a NumPy array to a NumPy array.