Python 代码片段收藏

list 列表相关

list 中最小值、最大值

import operator
 
values = [1, 2, 3, 4, 5]
 
min_index, min_value = min(enumerate(values), key=operator.itemgetter(1))
max_index, max_value = max(enumerate(values), key=operator.itemgetter(1))
 
print('min_index:', min_index, 'min_value:', min_value)
print('max_index:', max_index, 'max_value:', max_value)

# Out
min_index: 0 min_value: 1
max_index: 4 max_value: 5

list 中连续元素之间的差

from itertools import islice
ls = [1,2,3,5,8]
diff = [j-i for i,j in zip(ls, islice(ls, 1, None))]
print(diff)

# Out
[1, 1, 2, 3]

删除列表中的重复元素

下面这种方法不能维持顺序：

x = [1, 8, 4, 5, 5, 5, 8, 1, 8]
list(set(x))

# Out
[8, 1, 4, 5]

下面的方法，可以维持顺序：

from collections import OrderedDict
x = [1, 8, 4, 5, 5, 5, 8, 1, 8]
list(OrderedDict.fromkeys(x))

# Out
[1,8,4,5]

并行遍历2个列表

a = [1, 2, 3]
b = [4, 5, 6]

for (a_val, b_val) in zip(a, b):
    print(a_val, b_val)

# Out
1 4
2 5
3 6

合并列表值

输入的两个数组，输出一个是数组&值相加或者相乘：

# input
first = [1,2,3,4,5]
second = [6,7,8,9,10]

#output
three = [7,9,11,13,15]

# The zip function is useful here, used with a list comprehension.
# add
[x + y for x, y in zip(first, second)]

# other
[x*y for x, y in zip(first, second)]
[max(x,y) for x, y in zip(first, second)]

参考：

• Python中的常用代码

字典处理

字典做交、差、并

a={'name':'michael','age':"27",'sex':'male'}
b={'name':'hqh','age':'27'}
{k:a[k] for k in a.keys()-b.keys()}
out: {'sex': 'male'}
dict(a.items()-b.items())
out: {'name': 'michael', 'sex': 'male'}

需要注意的是，当字典的值有字典时，a.items()-b.items() 这种方式会报错 TypeError: unhashable type: 'dict'；

参考：

• cookbook-查找两字典的相同点
• 简书-Python 字典操作进阶

字典的Key与Value对调

m = {'A': 1, 'B': 2, 'C': 3}
invert_map_key_value = lambda m: dict(zip(m.values(), m.keys()))
invert_map_key_value(m)

# output: {1: 'A', 2: 'B', 3: 'C'}

参考：

• 2gua-几个Python代码片段

合并字典值

>>> from collections import Counter
>>> A = Counter({'a':1, 'b':2, 'c':3})
>>> B = Counter({'b':3, 'c':4, 'd':5})
>>> A + B
Counter({'c': 7, 'b': 5, 'd': 5, 'a': 1})

字符串相关

索引

tag='hx/mitaka_compute/12.0.0'
[m.start() for m in re.finditer('/',tag)]

参考：

• 在Python中查找所有出现的子字符串

将百分号的百分比字符串转为数字

p="75%"
float(p.strip('%'))/100

参考：

• Convert to numeric Python

剔除分隔符

通常做法：

''.join('A|B|C|D|E|F|G'.split('|'))

# output: 'ABCDEFG'

用 itertools.islice，因为可以节选字符串：

扫描二维码关注公众号，回复： 5397637 查看本文章

import itertools

''.join(itertools.islice('A|B|C|D|E|F|G', 6, None, 2))
# output: 'DEFG'

''.join(itertools.islice('A|B|C|D|E|F|G', 0, None, 2))
# output: ''ABCDEFG'

美观打印

import pprint as pp
animals = [{'animal': 'dog', 'legs': 4, 'breeds': ['Border Collie', 'Pit Bull', 'Huskie']}, {'animal': 'cat', 'legs': 4, 'breeds': ['Siamese', 'Persian', 'Sphynx']}]
pp.pprint(animals, width=1)

# Out
[{'animal': 'dog',
  'breeds': ['Border '
             'Collie',
             'Pit '
             'Bull',
             'Huskie'],
  'legs': 4},
 {'animal': 'cat',
  'breeds': ['Siamese',
             'Persian',
             'Sphynx'],
  'legs': 4}]

width参数指定一行上最大的字符数。设置width为1确保字典打印在单独的行

文件读写

基本文件读 txt

# Note: rb opens file in binary mode to avoid issues with Windows systems
# where 'rn' is used instead of 'n' as newline character(s).
 
 
# A) Reading in Byte chunks
reader_a = open("file.txt", "rb")
chunks = []
data = reader_a.read(64)  # reads first 64 bytes
while data != "":
    chunks.append(data)
    data = reader_a.read(64)
if data:
    chunks.append(data)
print(len(chunks))
reader_a.close()
 
 
# B) Reading whole file at once into a list of lines
with open("file.txt", "rb") as reader_b:   # recommended syntax, auto closes
    data = reader_b.readlines() # data is assigned a list of lines
print(len(data))
 
 
# C) Reading whole file at once into a string
with open("file.txt", "rb") as reader_c:
    data = reader_c.read() # data is assigned a list of lines
print(len(data))
 
 
# D) Reading line by line into a list
data = []
with open("file.txt", "rb") as reader_d:
    for line in reader_d:
        data.append(line)
print(len(data))

json 读写json文件

• json.loads()是将str转化成dict格式，json.dumps()是将dict转化成str格式。
• json.load()和json.dump()也是类似的功能，只是与文件操作结合起来了。

# 解码
import json
with open('build_info.json','r') as f:
    array = json.load(f)
print(array)

在编码JSON的时候，还有一些选项很有用。 如果你想获得漂亮的格式化字符串后输出，可以使用 json.dumps() 的indent参数。 它会使得输出和pprint() 函数效果类似：

>>> print(json.dumps(data))
{"price": 542.23, "name": "ACME", "shares": 100}
>>> print(json.dumps(data, indent=4))
{
    "price": 542.23,
    "name": "ACME",
    "shares": 100
}
>>>

保存为 json 文件：

# 编码
import json
a = {"name":"michael"}
with open("demo.json","w") as f:
    json.dump(a, f, indent=4)

• CookBook-6.2 读写JSON数据
• 简书-Python: json模块实例详解
• 官宣-json — JSON encoder and decoder
• Reading JSON file with Python 3
• What is the difference between json.load() and json.loads() functions in Python?

时间日期

基本时间（time）和日期（date）

import time
 
# print time HOURS:MINUTES:SECONDS
# e.g., '10:50:58'
print(time.strftime("%H:%M:%S"))
 
# print current date DAY:MONTH:YEAR
# e.g., '05/01/2019'
print(time.strftime("%d/%m/%Y"))

# Out
15:18:03
05/01/2019

字符串和日期的相互转换

strptime 是将字符串转换为 datetime，其实这个方法的全称是 “string parse time”，叫做字符串解析成时间，重点在解析（parse）:

from datetime import datetime

date_obj = datetime.strptime('2018-10-15 20:59:29', '%Y-%m-%d %H:%M:%S')
print(type(date_obj),date_obj)

# Out
<class 'datetime.datetime'> 2018-10-15 20:59:29

strftime 是将 datetime 转换为字符串，全称是 “string format time”，翻译过来就是将字符串的形式来格式化时间，重点在格式化（format），使之以一种可读的字符串形式返回：

from datetime import datetime
date_obj = datetime.now()
date_string = datetime.now().strftime("%Y-%m-%d %H:%M:%S")
print(type(date_string),date_string)

# Out
<class 'str'> 2019-01-05 18:41:04

参考：

• 3分钟学会一个小技巧

编码相关

Python Requests 编码问题

• requests github issue

下载

Python下载文件

• urlretrieve

Python根据url下载目录或者文件

    def download_package(self, package_url):
        print("start download_build_result")
        if not package_url.endswith("/"):
            package_url += '/'
        cmd = "wget -c -r -nd -np -P %s %s" % ("output", package_url)
        print(cmd)
        os.system(cmd)
        print(os.getcwd())
        print("finish download_build_result")

数据处理

Python Pandas处理Excel数据

逐行处理数据 iterrows

for idx, row in data.iterrows():
    project_name=row['projectName']
    tag_name=row['tagName']

Pandas追加模式写入csv文件

data = pd.DataFrame([[1,2,3]])
csv_headers=['A','B','C']
data.to_csv('./Marvel3_yingpping.csv', header=csv_headers, index=False, mode='a+', encoding='utf-8')
data = pd.DataFrame([[4,5,6]])
data.to_csv('./Marvel3_yingpping.csv', header=False, index=False, mode='a+', encoding='utf-8')
data = pd.DataFrame([[7,8,9]])
data.to_csv('./Marvel3_yingpping.csv', header=False, index=False, mode='a+', encoding='utf-8')

Python-CSV-Excel

• Python Pandas iterrows method

for idx, row in data.iterrows():
    project_name=row['projectName']
    tag_name=row['tagName']

to_csv表格中文乱码

ipython中直接打印df，中文没有乱码，但是to_csv方法存储时，中文有乱码。

df.to_csv('file.csv',encoding='utf-8-sig')

参考：

• Pandas df.to_csv(“file.csv” encode=“utf-8”) still gives trash characters for minus sign
• 关于pandas中,to_csv函数输出的utf8数据用Excel打开是乱码

itero

看题目：

• python数据处理，字典生成的一个问题答案中有位前辈用这个用的炉火纯青啊！

Shell/Linux 操作相关

Python运行shell命令的函数：

def run(cmd_str, fatal=True):
    # this is not a good implement
    log.command(log.term.cmd(cmd_str))
    ret = os.system(cmd_str)
    if ret is not 0:
        if fatal:
            log.error('[ERROR] run cmd: %s failed', cmd_str)
            os._exit(1)
        else:
            log.info('[INFO] %s is not fatal' % cmd_str)

调用外部的命令

# 
import subprocess

subprocess.call(['mkdir', 'empty_folder'])

# 运行一条命令并输出得到的结果
output = subprocess.check_output(['ls', '-l'])

# 上面的调用是阻塞的
# 如果运行shell中内置的命令，如cd或者dir，需要指定标记shell=True
output = subprocess.call(['cd', '/'], shell=True)

# 对于更高级的用例，可以使用 Popen constructor。

Python 3.5引进了一个新的run函数，它的行为与call和check_output很相似。如果你使用的是3.5版本或更高版本，看一看run的文档，里面有一些有用的例子。否则，如果你使用的是Python 3.5以前的版本或者你想保持向后兼容性，上面的call和check_output代码片段是你最安全和最简单的选择

参考：

计算文件的校验值

可以计算文件的 md5、sha256 等值

# https://pymotw.com/3/hashlib/index.html#module-hashlib
def get_verify_value(file_path, verify_type):
    """
    计算指定文件的校验值
    :param file_path: 文件路径
    :param verify_type: 校验值类型，md5 sha256 等等
    :return:
    """
    h = hashlib.new(verify_type)
    if not file_path:
        return None
    with open(file_path, 'rb') as f:
        for block in iter(lambda: f.read(4096), b""):
            h.update(block)
    return h.hexdigest()

性能相关

脚本的运行时间

import time
 
start_time = time.clock()
 
for i in range(10000000):
    pass
 
elapsed_time = time.clock() - start_time
print("Time elapsed: {} seconds".format(elapsed_time))

# Out
Time elapsed: 0.30121700000000007 seconds

import timeit
elapsed_time = timeit.timeit('for i in range(10000000): pass', number=1)
print("Time elapsed: {} seconds".format(elapsed_time))

# Out
Time elapsed: 0.2051873060000844 seconds

计算运行时间

class Timer(object):
    def __enter__(self):
        self.error = None
        self.start = time.time()
        return self
    def __exit__(self, type, value, tb):
        self.finish = time.time()
        if type:
            self.error = (type, value, tb)
    def duration(self):
        return self.finish - self.start
with Timer() as timer:
    func()
timer.duration()

# Out
0.29994797706604004

参考：

• CSDN- python代码片段

目录、路径相关

基本目录文件操作

import os
import shutil
import glob
 
# working directory
c_dir = os.getcwd()                 # show current working directory
os.listdir(c_dir)                   # shows all files in the working directory
os.chdir('~/Data')                  # change working directory
 
 
# get all files in a directory
glob.glob('/Users/sebastian/Desktop/*')
 
# e.g.,  ['/Users/sebastian/Desktop/untitled folder', '/Users/sebastian/Desktop/Untitled.txt']
 
# walk
tree = os.walk(c_dir)
# moves through sub directories and creates a 'generator' object of tuples
# ('dir', [file1, file2, ...] [subdirectory1, subdirectory2, ...]),
#    (...), ...
 
#check files: returns either True or False
os.exists('../rel_path')
os.exists('/home/abs_path')
os.isfile('./file.txt')
os.isdir('./subdir')
 
 
# file permission (True or False
os.access('./some_file', os.F_OK) # File exists? Python 2.7
os.access('./some_file', os.R_OK) # Ok to read? Python 2.7
os.access('./some_file', os.W_OK) # Ok to write? Python 2.7
os.access('./some_file', os.X_OK) # Ok to execute? Python 2.7
os.access('./some_file', os.X_OK | os.W_OK) # Ok to execute or write? Python 2.7
 
# join (creates operating system dependent paths)
os.path.join('a', 'b', 'c')
# 'a/b/c' on Unix/Linux
# 'a\b\c' on Windows
os.path.normpath('a/b/c') # converts file separators
 
 
# os.path: direcory and file names
os.path.samefile('./some_file', '/home/some_file')  # True if those are the same
os.path.dirname('./some_file')  # returns '.' (everythin but last component)
os.path.basename('./some_file') # returns 'some_file' (only last component
os.path.split('./some_file') # returns (dirname, basename) or ('.', 'some_file)
os.path.splitext('./some_file.txt') # returns ('./some_file', '.txt')
os.path.splitdrive('./some_file.txt') # returns ('', './some_file.txt')
os.path.isabs('./some_file.txt') # returns False (not an absolute path)
os.path.abspath('./some_file.txt')
 
 
# create and delete files and directories
os.mkdir('./test')  # create a new direcotory
os.rmdir('./test')  # removes an empty direcotory
os.removedirs('./test') # removes nested empty directories
os.remove('file.txt')   # removes an individual file
shutil.rmtree('./test') # removes directory (empty or not empty)
 
os.rename('./dir_before', './renamed') # renames directory if destination doesn't exist
shutil.move('./dir_before', './renamed') # renames directory always
 
shutil.copytree('./orig', './copy') # copies a directory recursively
shutil.copyfile('file', 'copy')     # copies a file
 
 
# Getting files of particular type from directory
files = [f for f in os.listdir(s_pdb_dir) if f.endswith(".txt")]
 
# Copy and move
shutil.copyfile("/path/to/file", "/path/to/new/file")
shutil.copy("/path/to/file", "/path/to/directory")
shutil.move("/path/to/file","/path/to/directory")
 
# Check if file or directory exists
os.path.exists("file or directory")
os.path.isfile("file")
os.path.isdir("directory")
 
# Working directory and absolute path to files
os.getcwd()
os.path.abspath("file")

参考：

• 斗大的熊猫-收集的有用的 Python 代码片段

## Python 删除文件夹

def onerror(func, path, exc_info):
    """
    Error handler for ``shutil.rmtree``.

    If the error is due to an access error (read only file)
    it attempts to add write permission and then retries.

    If the error is for another reason it re-raises the error.

    Usage : ``shutil.rmtree(path, onerror=onerror)``
    """
    import stat
    if not os.access(path, os.W_OK):
        # Is the error an access error ?
        os.chmod(path, stat.S_IWUSR)
        func(path)
    else:
        raise

参考：

• shutil.rmtree fails on Windows with 'Access is denied'

## Python 切换目录

执行完，返回之前目录

import contextlib
@contextlib.contextmanager
def cdir(path):
    prev_cwd = os.getcwd()
    os.chdir(path)
    try:
        yield
    finally:
        os.chdir(prev_cwd)

用法：

with cdir(path):
    func()

搜索指定目录下的文件

将指定目录及其子目录下的文件搜索出来：

def find_file(start_path, name):
    """
    search the files of name from the dir start_path，存放的是搜索文件的路径
    :param start_path: the search scope of dir
    :param name: the name of search file
    :return: set of files path
    """
    files_path = set()
    for rel_path, dirs, files in os.walk(start_path):
        # if name in files:
        for f in files:
            if name in f:
                full_path = os.path.join(start_path, rel_path, f)
                path = os.path.normpath(os.path.abspath(full_path))
                files_path.add(path)
    return files_path

只列出文件夹下的文件夹

[ name for name in os.listdir(thedir) if os.path.isdir(os.path.join(thedir, name)) ]

filter(os.path.isdir, os.listdir(os.getcwd()))

• How to list only top level directories in Python?

Python Path相关问题

os.path.split(r"C:\foo\bar\file_name.txt")

• 格式化文件路径

# 数据库

## MySQL 数据库

db = MySQLdb.connect("localhost","your_username","your_password","your_dbname")
cursor = db.cursor()
sql = "select Column1,Column2 from Table1"
cursor.execute(sql)
results = cursor.fetchall()
 
for row in results:
    print row[0]+row[1]
 
db.close()

参考：

• 斗大的熊猫-最常用的Python代码片段