Derivation of python

Derivation of python
Today, when looking at the company's batch deployment code (using fabric), I found a very interesting decorator, which uses the python derivation, especially to record it. The Code is as follows:

def host_restrict(tags=[], names=[]):    def decorator(f):        @functools.wraps(f)        def decorated(*args, **kwargs):            host_info = hosts.get(env.host)            if not host_info:                return f(*args, **kwargs)            if tags and host_info[0] not in tags:                raise StandardError('Tag %s not in %s' % (host_info[0], tags))            if names and not set(names).intersection(                    set([s for s in host_info[1].split(' ') if s.strip()])):                raise StandardError(                    'Name %s not in %s' % (host_info[1], names))            return f(*args, **kwargs)        return decorated    return decorator

 

Note: This decorator is mainly used to determine whether the user-entered tag and names meet the requirements. For example: # The host_info format is host_info = ['api ', 'nginx'] @ host_restrict (['api '], ['nginx']) def init ():... okay, don't pay attention to the details. There is a line of code that attracts my attention. set ([s for s in host_info [1]. split ('') if s. strip ()]) is a list derivation of python ,. The derivation is a powerful and popular feature in python. It has the advantages of concise language and fast speed. Derivation includes list derivation dictionary Derivation Set derivation nested list derivation NOTE: dictionary and set derivation were recently added to Python (Python 2.7 and Python 3.1 and later versions ). the following is a brief introduction: [LIST derivation] list derivation can create a new list in a very concise manner: only a concise expression can be used to convert and deform the elements. The basic format is as follows: [expr for value in collectionif condition] the filtering condition is optional. Depending on the actual application, only the expression is left, which is equivalent to the following for loop: result = [] forvalue in collection: ifcondition: result. append (expression) sample code :#! /Usr/bin/env python #-*-coding: UTF-8-*-# Example 1: filter out the list of strings whose length is less than 3, and convert the rest into uppercase letters names = ['bob', 'Tom ', 'Alice', 'Jerry ', 'wendy', 'Smith '] print [name. upper () for name in names if len (name)> 3] # Example 2: print the number of partitions that can be divisible by 3. print [x * x for x in range (10) if x % 3 = 0] Output: ['Alice ', 'Jerry', 'wendy ', 'Smith'] [0, 9, 36, 81] [dictionary derivation] The dictionary and set derivation are the continuation of this idea. The syntax is similar, but only the set and dictionary are generated. The basic format is as follows: {key_expr: value_expr for value in collection if condition} sample code :#! /Usr/bin/env python #-*-coding: UTF-8-*-# Example 1: Use dictionary derivation-create a dictionary strings = ['import' with a string and its length ', 'Is ', 'with', 'if', 'file', 'exception'] print {key: len (key) for val, key in enumerate (strings)} output: {'exception': 9, 'is: 2, 'file': 4, 'import': 6, 'with': 4, 'if ': 2} [set derivation] The set derivation is very similar to the list derivation. The only difference is that {} is used instead of []. The basic format is as follows: {expr for value incollection if condition} sample code :#! /Usr/bin/env python #-*-coding: UTF-8-*-# Example 1: Use a set to derive the strings = ['A', 'is ', 'with', 'if', 'file', 'exception'] print {len (s) for s in strings} output: set ([1, 2, 4, 9]) [nested list derivation] A nested list refers to a list of nested lists, such as: L = [[, 3], [, 6, 9] sample code: # Example 1: list comprehension replace nested loop print [[x, y] for x in range (3) for y in range (6)] # equivalent to z = [] for x in range (3): for y in range (6): z. append ([x, y]) print z # Example 2: A nested list composed of a list of men and women. The name contains two or more letters e, component list # implemented using a for Loop:

tmp = []names = [    ['Tom','Billy','Jefferson','Andrew','Wesley','Steven','Joe'],    ['Alice','Jill','Ana','Wendy','Jennifer','Sherry','Eva']]for lst in names:    for name in lst:        if name.count('e') >= 2:            tmp.append(name)print tmp

 

# Implement with nested list: print [name for lst in names for name in lst if name. count ('E')> = 2] Output: ['jefferson ', 'westsley', 'steven ', 'jennifer'] ['jefferson ', 'westsley ', 'steven ', 'jennifer']