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List Comprehensions

Метки: python

A list comprehension consists of the following parts:

• An Input Sequence.
• A Variable representing members of the input sequence.
• An Optional Predicate expression.
• An Output Expression producing elements of the output list from members of the Input Sequence that satisfy the predicate.

Say we need to obtain a list of all the integers in a sequence and then square them:

a_list = [1, ‘4’, 9, ‘a’, 0, 4]

squared_ints = [ e**2 for e in a_list if type(e) == types.IntType ]

print squared_ints
# [ 1, 81, 0, 16 ]

• The iterator part iterates through each member e of the input sequence a_list.
• The predicate checks if the member is an integer.
• If the member is an integer then it is passed to the output expression, squared, to become a member of the output list.

Much the same results can be achieved using the built in functions, map, filter and the anonymous lambda function.

The filter function applies a predicate to a sequence:

filter(lambda e: type(e) == types.IntType, a_list)

Map modifies each member of a sequence:

map(lambda e: e**2, a_list)

The two can be combined:

map(lambda e: e**2, filter(lambda e: type(e) == types.IntType, a_list))

The above example involves function calls to map, filter, type and two calls to lambda. Function calls in Python are expensive. Furthermore the input sequence is traversed through twice and an intermediate list is produced by filter.

The list comprehension is enclosed within a list so, it is immediately evident that a list is being produced. There is only one function call to type and no call to the cryptic lambda instead the list comprehension uses a conventional iterator, an expression and an if expression for the optional predicate.