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Python Gotchas¶

What is the output of the following function? Why?

::

>>> def f(x):
...     print("3*x = %s" % 3*x)
>>> f(5)
3*x = 33*x = 33*x = 33*x = 33*x = 3

Answer:

Look at the operator precedence: % has higher precedence that *, so the print statement is parsed as print(("3*x = %s" % 3)*x) which means, print x copies of the string. The fix is recommended in the manual: always use a tuple after %: print("3*x = %s" % (3*x,))