reading-notes

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List Comprehensions in Python

• List comprehensions : provide a concise way to create lists.

• It consists of brackets containing an expression followed by a for clause, then zero or more for or if clauses. The expressions can be anything, meaning you can put in all kinds of objects in lists.

• The result will be a new list resulting from evaluating the expression in the context of the for and if clauses which follow it.

• The list comprehension always returns a result list.

• The list comprehension starts with a ‘[‘ and ‘]’, square brackets, to help you remember that the result is going to be a list.

previously we used to populate lists like this

            new_list = []
            for i in old_list:
              if filter(i):
                new_list.append(expressions(i))

you can do it in a cleaner better way using list comprehensions

new_list = [expression(i) for i in old_list if filter(i)

• new_list The new list (result).

• expression(i) Expression is based on the variable used for each element in the old list.

• for i in old_list The word for followed by the variable name to use, followed by the word in the old list.

• if filter(i) Apply a filter with an If-statement.