An introduction to the derivation in Python

The derivation is a very powerful and popular feature in Python, with the advantages of simple language and fast speed. The derivation formula includes:

1. List-derived
2. Dictionary derivation type
3. Set Deduction formula
4. Nested-List derivation

Note: dictionaries and set derivations are only recently added to Python (Python 2.7 and Python version 3.1 or later). The following is a brief introduction:

" List-derived "

List derivation can be very concise to construct a new list: The resulting element can be transformed with a simple expression

Its basic format is as follows:

[Expr for value in collection ifcondition]

Filter conditions are optional, depending on the actual application, leaving only the expression, equivalent to the following for loop:

result = []for value in collection:    if Condition:        result.append (expression)

Example 1: Filter out a list of strings that are less than 3 in length and convert the rest to uppercase letters

>>> names = [' Bob ', ' Tom ', ' Alice ', ' Jerry ', ' Wendy ', ' Smith ']>>> [Name.upper () for name in names if Len ( Name) >3][' ALICE ', ' JERRY ', ' WENDY ', ' SMITH ']

【Dictionary Derivation type】

Dictionary and set derivation is the continuation of the idea, the syntax is similar, but the result is a set and a dictionary. Its basic format is as follows:

{ key_expr:value_expr for value in collection if condition }

Example 1: using a dictionary derivation to build a dictionary with a string and its length

>>> strings = [' Import ', ' is ', ' was ', ' with ', ' if ', ' file ', ' exception ']>>> D = {Key:val to Val,key in enumerate ( strings)}>>> d{' exception ': 5, ' is ': 1, ' file ': 4, ' import ': 0, ' with ': 2, ' If ': 3}

" Set Deduction Formula "

The set deduction is very similar to the list derivation, except that the {} is the only difference. Its basic format is as follows:

{ expr for value in collection if condition }

Example 1: building a set of string lengths with set derivation

>>> strings = [' A ', ' is ', ' with ', ' if ', ' file ', ' exception ']>>> {len (s) for s in Strings}    # Having the same length will only leave one, which in fact is also very useful set ([1, 2, 4, 9])

" nested list derivation "

Nested lists are nested lists in a list, such as:

>>> L = [[4,5,6], [[         7,8,9]]

Example 1: a nested list of men's lists and women's lists, with names containing more than two letters E in their names, to form a list

names = [[' Tom ', ' Billy ', ' Jefferson ', ' Andrew ', ' Wesley ', ' Steven ', ' Joe '],         [' Alice ', ' Jill ', ' Ana ', ' Wendy ', ' Jennifer ', ' Sherry ', ' Eva ']

Implemented with A For loop:

TMP = []for lst in Names: for    name in LST:        if Name.count (' E ') >= 2:            tmp.append (name) print tmp# output >>& Gt [' Jefferson ', ' Wesley ', ' Steven ', ' Jennifer ']

implemented with nested lists:

>>> names = [[' Tom ', ' Billy ', ' Jefferson ', ' Andrew ', ' Wesley ', ' Steven ', ' Joe '],         [' Alice ', ' Jill ', ' Ana ', ' Wendy ', ' Jennifer ', ' Sherry ', ' Eva ']]>>> [name for LST in names for name in LST if Name.count (' E ') >=2]  #注意遍历 Order, which is the key to implementation [' Jefferson ', ' Wesley ', ' Steven ', ' Jennifer ']

An introduction to the derivation in Python