Fluent Python

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String

String Indexing

a = 'Hello world'
b = a[0]          # 'H'
c = a[4]          # 'o'
d = a[-1]         # 'd' (end of string)

d = a[:5]     # 'Hello'
e = a[6:]     # 'world'
f = a[3:8]    # 'lo wo'
g = a[-5:]    # 'world'

String operations

# Concatenation (+)
a = 'Hello' + 'World'   # 'HelloWorld'
b = 'Say ' + a          # 'Say HelloWorld'

# Length (len)
s = 'Hello'
len(s)                  # 5

# Membership test (`in`, `not in`)
t = 'e' in s            # True
f = 'x' in s            # False
g = 'hi' not in s       # True

# Replication (s * n)
rep = s * 5             # 'HelloHelloHelloHelloHello'

String Mutability

Strings are "immutable" or read-only. Once created, the value can't be changed. All operations and methods that manipulate string data, always create new strings.

String Conversions

str()

Default Parameter Values in Python

>>> def function(data=[]):
...     data.append(1)
...     return data
...
>>> function()
[1]
>>> function()
[1, 1]
>>> function()
[1, 1, 1]

>>> id(function())
12516768
>>> id(function())
12516768
>>> id(function())
12516768

# function() 函数在不同函数调用中一直在使用同一个列表对象
# instances 共享了 __dict__ 里面的默认参数，