Python Tips¶

Just some python tips by lzhbrian

Basic¶

Basic - eval, exec¶

An explanation

http://www.mojidong.com/python/2013/05/10/python-exec-eval/

Basic - 基本运算¶

python 除法

# python2
/ 3 = 3
/ 3.0 = 3.333333
// 3 = 3
// 3.0 = 3.0 # 截取小数部分但还是小数

# python3
/ 3 = 3.333333
/ 3.0 = 3.333333
// 3 = 3
// 3.0 = 3.0 # 截取小数部分但还是小数

查看变量类型
```
type()
```

ord, chr, unichr

chr(65) = 'A'
ord('a') = 97
unichr(12345) = u'\u3039'

Bit manipulation¶

Bit manipulation - 进制转换¶

10进制转换成2、8、16进制

bin();oct();hex()
60 = 0b11100 = 0o74 = 0x3c

其他进制转换成10进制
```
int('101',2) = 5
int('17',8) = 15
```

Bit manipulation - 位操作¶

二进制操作
- &：按位与
- |：按位或
- ^：按位异或
- ~：取反
- <<：左移
- >>: 右移

List¶

List - Sort¶

Two method

a.sort() # did on a
a = sorted(a, reverse = True) # gen a new item

List - Counter¶

count 函数

a = [1,1,1,'1']
a.count(1) = 3
a.count('1') = 1

collections.Counter 模块计数

http://www.zlovezl.cn/articles/collections-in-python/

from collections import Counter
s = '''A Counter i....'''.lower()

c = Counter(s)
# 获取出现频率最高的5个字符
print c.most_common(5)

# Result:
[(' ', 54), ('e', 32), ('s', 25), ('a', 24), ('t', 24)]

List - Join, Split¶

http://wangwei007.blog.51cto.com/68019/1100587

li = ['my','name','is','bob']
'_'.join(li) = 'my_name_is_bob'
'a  b'.split(' ') = ['a','','b'] # 中间两个空格

List - Extended Slices¶

Official explanation

https://docs.python.org/2/whatsnew/2.3.html#extended-slices

L = range(11) # L = [0,1,2,3,4,5,6,7,8,9,10]
L[::2] # = [0,2,4,6,8,10]
L[0:10:2] # = [0,2,4,6,8]
L[::-1] # L = [10,9,8,7,6,5,4,3,2,1,0]

List - Special Usage¶

获取特定index范围

# list
a = [1,2,3,4,5,6]
a[:3] = [1,2,3]
a[3:] = [4,5,6]

获取数组最后一个元素 Last element
```
a = [1,2,3,4]
a[-1] = 4
```
反转 Reverse
```
a = [1,2,3]
a[::-1] = [3,2,1]
```

range 用法

range(10) == range(0,10) == range(0,10,1) == [0,1,2,3,4,5,6,7,8,9]
range(10,0,-1) == [10,9,8,7,6,5,4,3,2,1] == range(1,11)[::-1]

Copy

a = [1,2,3,4,5]
b = a
b.append(6)
print a # [1,2,3,4,5,6]

b = a[:]
b.append(6)
print a # [1,2,3,4,5]

List - Bisect¶

Official Intro: https://docs.python.org/2/library/bisect.html
An amazing module, can automatically insert a value into a sorted list
- Note: The list must be sorted from small to big

插入：

import bisect as bi

# insert to the left
bi.insort_left(l, val) # bi.insort_left(l, val, lo=0, hi=len(l))
l.insert(bi.bisect_left(l, val), val) # same as above

# Below is equivalent, insert to the right
bi.insort_right(l, val)
bi.insort(l, val)

查找插入的位置

import bisect as bi

# return the index of 'will insert place' (left)
bi.bisect_left(l, val) # bi.bisect_left(l, val, lo=0, hi=len(l))

# Below is equivalent, return the index of 'will insert place' (right)
bi.bisect_right(l, val)
bi.bisect(l, val)

Dict¶

Dict - 遍历 Traverse¶

遍历：
```
for k,v in d.items():
  print k,v
```

Dict - 排序 Sort¶

Using operator

# Sort by keys
import operator
x = {1: 2, 3: 4, 4: 3, 2: 1, 0: 0}
sorted_x = sorted(x.items(), key=operator.itemgetter(0),reverse=True)
for k,v in sorted_x: # Return a tuple, can directly use: 'for k,v in sorted_x'
    ...

# Sort by values:
import operator
x = {1: 2, 3: 4, 4: 3, 2: 1, 0: 0}
sorted_x = sorted(x.items(), key=operator.itemgetter(1),reverse=True)

Using lambda

# Sort by keys:
sorted(d.items(), key=lambda x: x[0],reverse=True)

# Sort by values:
sorted(d.items(), key=lambda x: x[1],reverse=True)

Set¶

Set - Basic Usage¶

Basic

x.add('d') # 添加一项 
x.update([10,37,42]) # 在s中添加多项
x.remove('H')
x in s # 测试 x 是否是 s 的成员
x not in s # 测试 x 是否不是 s 的成员

symbol

x & y # 交集
x | y # 并集
x - y # 差集
x ^ y # 对称差集（项在x或sy中，但不会同时出现在二者中）

Others

# 测试是否 s 中的每一个元素都在 t 中
s.issubset(t) == True
s <= t

# 测试是否 t 中的每一个元素都在 s 中
s.issuperset(t)
s >= t

# 删除 set s 中的所有元素
s.clear()

File¶

File - Read csv¶

# First Row is column name
with open(path, 'r') as f:
    reader = csv.DictReader(f)
    # fields: No., Time, Source, Destination, Protocol, Length, Host, Info, Request URI Query Parameter
    for row in reader:
        print row["Info"]

# First Row is data, need to specify column name
with open('names.csv', 'w') as csvfile:
    fieldnames = ['first_name', 'last_name']
    writer = csv.DictWriter(csvfile, fieldnames=fieldnames)

# Does not use column name, row is a list instead
with open('some.csv', 'rb') as f:
    reader = csv.reader(f)
    for row in reader:
        print row

Special¶

Special - with¶

with open("x.txt") as f:
    data = f.read()
    # do something with data

with open("x.txt") as f1, open('xxx.txt') as f2:
    # do something with f1,f2

Special - Levenshtein Distance¶

Installation:

Ref: https://github.com/ztane/python-Levenshtein/

Ref: http://www.cnblogs.com/kaituorensheng/archive/2013/05/18/3085653.html
```
pip install python-Levenshtein
```

Usage:

import Levenshtein

# Calc Levenshtein dDistance (or Edit Distance)
# 删除、插入、替换 +1
Levenshtein.distance(str1, str2)

# Calc Levenshtein Ratio, r = (sum - ldist) / sum
# sum = len(str1) + len(str2)
# but 删除、插入 +1，替换 +2
Levenshtein.ratio(str1, str2)

# Calc Hamming Distance
# len must be the same
Levenshtein.hamming(str1, str2)

# Calc Jaro Distance
Levenshtein.jaro(s1, s2)

# Calc Jaro–Winkler Distance
Levenshtein.jaro_winkler(s1, s2)