Delete item in a list in Python

Use the list.remove() method to remove the first occurrence, in-place:

a.remove((1, 2))

Demo:

>>> a = [(4, 5), (1, 2), (7, 5), (1, 2), (5, 2)]
>>> a.remove((1, 2))
>>> a
[(4, 5), (7, 5), (1, 2), (5, 2)]

See the Mutable Sequence Types documentation:

s.remove(x)
same as del s[s.index(x)]

and s.index() only ever finds the first occurrence.