tips and tricks

**1. 将两个字典合并为一个字典**
- update
```bash
In [1]: d1 = {'a':1, 'b':2}
In [2]: d2 = {'c':3, 'd':4}
In [3]: d1.update(d2)
In [4]: d1
Out[4]: {'a': 1, 'b': 2, 'c': 3, 'd': 4}
```
- ** 解包
```bash
In [1]: d1 = {'a':1, 'b':2}
In [2]: d2 = {'c':3, 'd':4}
In [3]: dict(**d1, **d2)
Out[3]: {'a': 1, 'b': 2, 'c': 3, 'd': 4}
```
**2. 去重**
- 列表元素去重
```bash
In [1]: lst = [5, 2, 2, 1]
In [2]: set(lst)
Out[2]: {1, 2, 5}
```
- 列表元素去重且保持原来的顺序
```bash
a = [5, 5, 6, 2, 2, 1, 4, 4]
b = []
for x in a:
    if x not in b:
        b.append(x)
print(b) # [5, 6, 2, 1, 4]
```
**3. 找出两列表中不同的元素**
```bash
a = [1,2,3,4,5,6]
b = [1,2,3,5]
# 列表解析式
c = [x for x in a if x not in b] # [4, 6]
# 转成元组，使用差集
d = set(a) - set(b) # {4, 6}
```
**4. Counter计数对象:dict类的子类**
```bash
>>> from collections import Counter
>>> a = Counter('hello world')
>>> a
Counter({'l': 3, 'o': 2, 'h': 1, 'e': 1, ' ': 1, 'w': 1, 'r': 1, 'd': 1})
>>> a.most_common(2)
[('l', 3), ('o', 2)]
>>> a.items()
dict_items([('h', 1), ('e', 1), ('l', 3), ('o', 2), (' ', 1), ('w', 1), ('r', 1), ('d', 1)])
```
**5. 格式化打印**
```bash
>>> data = {'name':'tom', 'age':18, 'gender':'male', 'other':[{'hobbies':'playing games','fruits':'apple'},{'name':'jerry', 'age':'18', 'relationship':'friends'}]}
>>> import json
print(json.dumps(data, indent=2))
>>> print(json.dumps(data, indent=2))
{
  "name": "tom",
  "age": 18,
  "gender": "male",
  "other": [
    {
      "hobbies": "playing games",
      "fruits": "apple"
    },
    {
      "name": "jerry",
      "age": "18",
      "relationship": "friends"
    }
  ]
}
>>> from pprint import pprint
>>> pprint(data)
{'age': 18,
 'gender': 'male',
 'name': 'tom',
 'other': [{'fruits': 'apple', 'hobbies': 'playing games'},
           {'age': '18', 'name': 'jerry', 'relationship': 'friends'}]}
```
**6. 元素组合方式**
```bash
>>> from itertools import combinations
>>> lst = ['a', 'b', 'c', 'd']
>>> for item in combinations(lst, 2):
...     print(item)
... 
('a', 'b')
('a', 'c')
('a', 'd')
('b', 'c')
('b', 'd')
('c', 'd')
```

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