file类使用出错，怎么回事？

def TT():
    poem = '''\
    Programming is fun
    When the work is done
    if you wanna make your work also fun:
        use Python!
    '''

    f = file('poem.txt', 'w') # open for 'w'riting
    f.write(poem) # write text to file
    f.close() # close the file

    f = file('poem.txt')
    # if no mode is specified, 'r'ead mode is assumed by default
    while True:
        line = f.readline()
        if len(line) == 0: # Zero length indicates EOF
            break
        print (line),
        # Notice comma to avoid automatic newline added by Python
    f.close() # close the file
  

>>> TT()
Traceback (most recent call last):
  File "<pyshell#30>", line 1, in <module>
    TT()
  File "<pyshell#29>", line 9, in TT
    f = file('poem.txt', 'w') # open for 'w'riting
NameError: global name 'file' is not defined