Updating and inserting mutable types of nested data types should be careful & do not use mutable types such as lists, dictionaries, etc. as the default parameters of functions

Question 1: Pay attention to nested data types that are inserted into mutable types

Look at a piece of code

if __name__ == '__main__':
    tx1 = {'from': 'Jason', 'to': 'Karmen'}
    tx2 = {'from': 'Jason', 'to': 'Karmen'}
    # id(txs[0]) = id(tx1) & id(txs[1]) = id(tx2)
    txs = [tx1, tx2]
    # [{'to': 'Karmen', 'from': 'Jason'}, {'to': 'Karmen', 'from': 'Jason'}]
    print txs

    # 往tx1新增一个键，txs也会更新
    tx1['?'] = '?'
    tx1['from'] = 'God'
    # [{'to': 'Karmen', 'from': 'God', '?': '?'}, {'to': 'Karmen', 'from': 'Jason'}]
    print txs

    mylist = []
    # id(mylist[0]) = id(tx1)
    mylist.append(tx1)
    mylist[0]['from'] = 'SKY'
    # [{'to': 'Karmen', 'from': 'SKY', '?': '?'}, {'to': 'Karmen', 'from': 'Jason'}]
    print txs
    # [{'to': 'Karmen', 'from': 'SKY', '?': '?'}]
    print mylist

    # id(mylist[0]) = id(mylist[1]) = id(tx1)
    mylist.append(tx1)
    mylist[1]['to'] = 'RAINBOW'
    # [{'to': 'RAINBOW', 'from': 'SKY', '?': '?'}, {'to': 'Karmen', 'from': 'Jason'}]
    print txs
    # [{'to': 'RAINBOW', 'from': 'SKY', '?': '?'}, {'to': 'RAINBOW', 'from': 'SKY', '?': '?'}]
    print mylist

The result is:

[{'to': 'Karmen', 'from': 'Jason'}, {'to': 'Karmen', 'from': 'Jason'}]
[{'to': 'Karmen', 'from': 'God', '?': '?'}, {'to': 'Karmen', 'from': 'Jason'}]
[{'to': 'Karmen', 'from': 'SKY', '?': '?'}, {'to': 'Karmen', 'from': 'Jason'}]
[{'to': 'Karmen', 'from': 'SKY', '?': '?'}]
[{'to': 'RAINBOW', 'from': 'SKY', '?': '?'}, {'to': 'Karmen', 'from': 'Jason'}]
[{'to': 'RAINBOW', 'from': 'SKY', '?': '?'}, {'to': 'RAINBOW', 'from': 'SKY', '?': '?'}]

Problem 2: Don't use mutable types such as lists, dictionaries, etc. as default parameters of functions

Look at a piece of code

def f(x,l=[]):
    for i in range(x):
        l.append(i*i)
    print(l)

print('---1---')
f(4)
print('---2---')
f(5)

Results of the:

---1---
[0, 1, 4, 9]
---2---
[0, 1, 4, 9, 0, 1, 4, 9, 16]

The expected result is:

---1---
[0, 1, 4, 9]
---2---
[0, 1, 4, 9, 16]

problem explanation

When the function is defined, the value of the default parameter list in the function is saved, that is, the id of the list is saved.

1. When the list is inserted for the first time, the id of the list remains unchanged and the content changes.
2. In the second call, the same id is still taken, so inserting into the same list again will continue to insert on the basis of the first call.

problem solved

def f(x,l=None):
    if l is None:
        l = []
    for i in range(x):
        l.append(i*i)
    print(l)


print('---1---')
f(4)
print('---2---')
f(5)
print('---3---')
f(6)

result:

---1---
[0, 1, 4, 9]
---2---
[0, 1, 4, 9, 16]
---3---
[0, 1, 4, 9, 16, 25]