Subtract a value from every number in a list in Python?

With a list comprehension:

a = [x - 13 for x in a]

If are you working with numbers a lot, you might want to take a look at NumPy. It lets you perform all kinds of operation directly on numerical arrays. For example:

>>> import numpy
>>> array = numpy.array([49, 51, 53, 56])
>>> array - 13
array([36, 38, 40, 43])

You can use map() function:

a = list(map(lambda x: x - 13, a))