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pythonpython-2.7indexingtupleselement

Why is a tuple of tuples of length 1 not actually a tuple unless I add a comma?

Given a tuple of tuples T:

(('a', 'b'))

and an individual tuple t1:

('a','b')

why does:

t1 in T

return False?

UPDATE: From Ipython:

In [22]: T = (('a','b'))

In [23]: t1 = ('a','b')

In [24]: t1 in T
Out[24]: False

And how then to check that a tuple is in another tuple?

Solution

  • The problem is because T is not a tuple of tuples, it is just a tuple. The comma makes a tuple, not the parentheses. Should be:

    >>> T = (('a','b'),)
>>> t1 = ('a', 'b')
>>> t1 in T
True

    In fact, you can loose the outer parentheses:

    >>> T = ('a','b'),
>>> t1 = 'a','b'
>>> type(T)
<type 'tuple'>
>>> type(T[0])
<type 'tuple'>
>>> type(t1)
<type 'tuple'>
>>> t1 in T
True

    Although sometimes they are needed for precedence, if in doubt put them in. But remember, it is the comma that makes it a tuple.