python中的常用操作

真的很常用诶.

对dict排序

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>>> from collections import OrderedDict

>>> # regular unsorted dictionary
>>> d = {'banana': 3, 'apple':4, 'pear': 1, 'orange': 2}

>>> # dictionary sorted by key -- OrderedDict(sorted(d.items()) also works
>>> OrderedDict(sorted(d.items(), key=lambda t: t[0]))
OrderedDict([('apple', 4), ('banana', 3), ('orange', 2), ('pear', 1)])

>>> # dictionary sorted by value
>>> OrderedDict(sorted(d.items(), key=lambda t: t[1]))
OrderedDict([('pear', 1), ('orange', 2), ('banana', 3), ('apple', 4)])

>>> # dictionary sorted by length of the key string
>>> OrderedDict(sorted(d.items(), key=lambda t: len(t[0])))
OrderedDict([('pear', 1), ('apple', 4), ('orange', 2), ('banana', 3)])

简单点:

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d = {2:3, 1:89, 4:5, 3:0}
sd = sorted(d.items())

for k,v in sd:
    print k, v
'''
1 89
2 3
3 0
4 5
'''

统计list中元素个数

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In [1]: from collections import Counter

In [2]: a = [1,2,34,3,2,5,6,1,1]

In [3]: Counter(a)
Out[3]: Counter({1: 3, 2: 2, 3: 1, 5: 1, 6: 1, 34: 1})

那些解析

解析json字符串

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import json
j = json.loads('{"one" : "1", "two" : "2", "three" : "3"}')
print j['two']

写入和读取python数据结构到文件:

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# Writing JSON data
with open('data.json', 'w') as f:
    json.dump(data, f)

# Reading data back
with open('data.json', 'r') as f:
    data = json.load(f)

对于中文,如果想写入文件时不进行编码,可以指定ensure_ascii=False

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json.dump(obj,f,ensure_ascii=False)

python里计时

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import time

start = time.time()
print("hello")
end = time.time()
print(end - start)

slice dict

dict本来不能像list那样slice,但itertools可以:

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In [14]: a = {'a':1,'b':2}

In [15]: a[:]
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-15-6ced218e2c37> in <module>()
----> 1 a[:]

TypeError: unhashable type: 'slice'

In [16]: import itertools

In [17]: dict(itertools.islice(a.items(),1))
Out[17]: {'a': 1}

检查NAN

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>>> import math
>>> x=float('nan')
>>> math.isnan(x)
True

快速清除文件内容

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open('file.txt','w).close()

列出文件夹内容与正则过滤

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In [1]: import os

In [2]: os.listdir('./')
Out[2]:
['0307_nodes.txt',
 '0307_links.txt',
 '0901_nodes.txt',
 '0901_links.txt'
]

其实python提供有过滤的库函数fnmatch,glob等

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path = "/home/relation-data-2015/"
files = fnmatch.filter(os.listdir(path),'*_single_links.txt')
files
'''
['0101_single_links.txt',
 '0901_single_links.txt',
 '0201_single_links.txt',
 '0307_single_links.txt',
 '0402_single_links.txt',
 '0215_single_links.txt',
 '0408_single_links.txt',
 '0501_single_links.txt']
'''

sorted函数

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In [4]: l = [{'a':1},{'a':4},{'a':2}]

In [5]: sorted(l,key=lambda d:d['a'])
Out[5]: [{'a': 1}, {'a': 2}, {'a': 4}]

注意,这并不会改变原有列表:

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In [6]: l
Out[6]: [{'a': 1}, {'a': 4}, {'a': 2}]

In [7]: l = sorted(l,key=lambda d:d['a'])

In [8]: l
Out[8]: [{'a': 1}, {'a': 2}, {'a': 4}]

修改dict的key值

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In [1]: d = {'女': 176, '男': 517}

In [2]: d['female'] = d.pop('女')

In [3]: d
Out[3]: {'female': 176, '男': 517}

取得dict的第一个key

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# way 1
my_dict = {'foo': 'bar'}
next(iter(my_dict)) # outputs 'foo'

# way 2
list(my_dict.keys())[0]

取得list里最频繁项

stackoverflow-python-most-common-element-in-a-list

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# way 1
from collections import Counter

def Most_Common(lst):
    data = Counter(lst)
    return data.most_common(1)[0][0]

# way 2
def most_common(lst):
    data = Counter(lst)
    return max(lst, key=data.get)

# way 3
>>> from statistics import mode
>>> mode([1, 2, 2, 3, 3, 3, 3, 3, 4, 5, 6, 6, 6])
3

# way 4
def most_common(lst):
    return max(set(lst), key=lst.count)

删除连续空格只保留一个

  1. 用2个空格替换成一个空格,需要循环替换
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    a = '1    3   5'
    while a.find('  ') != -1:
        a = a.replace('  ', ' ')
  2. 先分开再用join
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    ' '.join(s.split())

    日期操作

    获得年月日
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    from datetime import datetime
    today = datetime.today()
    today.day/month/year
    计算日期之间的月份差
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    from datetime import datetime,date

    def month_delta(start_date, end_date=datetime.now().date()):
        """
        返回 end_date  - start_date  的差值
            :param start_date:
            :param end_date:
            :return:  month_delta   int
        """
        flag = True
        if start_date > end_date:
            start_date, end_date = end_date, start_date
            flag = False
        year_diff = end_date.year - start_date.year
        end_month = year_diff * 12 + end_date.month
        delta = end_month - start_date.month
        # return -delta if flag is False else delta
        return abs(delta)