>>> l = list()
>>> l
Out[1]: []
>>> l = [1]
>>> l
Out[1]: [1]

# cannot add a number to a list
>>> l += 1
Traceback (most recent call last):
  File "/Users/guestuser/Library/Enthought/Canopy_64bit/User/lib/python2.7/site-packages/IPython/core/interactiveshell.py", line 2721, in run_code
    exec code_obj in self.user_global_ns, self.user_ns
  File "<ipython-input-1-bd21f9b992ad>", line 1, in <module>
    l += 1
TypeError: 'int' object is not iterable

# must add compatible types
>>> l += [1]
>>> l
Out[1]: [1, 1]
>>> l += (1)
>>> l
Out[1]: [1, 1, 1]

# can append a number
>>> l.append(1)
>>> l
Out[1]: [1, 1, 1, 1]

# appending a list does something different
>>> l.append([1])
>>> l
Out[1]: [1, 1, 1, 1, [1]]
# must add a list of lists to get the same result
>>> l += [[1]]
>>> l
Out[1]: [1, 1, 1, 1, [1], [1]]