Python 中的str是一种不可变序列，由 Unicode 码点组成。
这些码点可以包括字母、变音符号、定位字符、数字、货币符号、emoji、标点、空格和换行符等等。
由于不可变，str对象在内存中的值不会改变；那些看起来会修改字符串的方法，实际返回的是该str对象的新副本或新实例。
str字面量可以用单引号'或双引号"声明。需要时可以使用转义字符\。
>>> single_quoted = 'These allow "double quoting" without "escape" characters.'
>>> double_quoted = "These allow embedded 'single quoting', so you don't have to use an 'escape' character."
>>> escapes = 'If needed, a \'slash\' can be used as an escape character within a string when switching quote styles won\'t work.'
多行字符串用'''或"""声明。
>>> triple_quoted = '''Three single quotes or "double quotes" in a row allow for multi-line string literals.
Line break characters, tabs and other whitespace are fully supported.
You\'ll most often encounter these as "doc strings" or "doc tests" written just below the first line of a function or class definition.
They\'re often used with auto documentation ✍ tools.
'''
字符串可以用+运算符拼接。
这种方法应尽量少用，因为它性能不佳，也不容易维护。
language = "Ukrainian"
number = "nine"
word = "дев'ять"
sentence = word + " " + "means" + " " + number + " in " + language + "."
>>> print(sentence)
...
"дев'ять means nine in Ukrainian."
如果需要把list、tuple、set或其他由单个字符串组成的集合合并成一个str，<str>.join(<iterable>)是更好的选择：
# str.join() makes a new string from the iterables elements.
>>> chickens = ["hen", "egg", "rooster"] # Lists are iterable.
>>> ' '.join(chickens)
'hen egg rooster'
# Any string can be used as the joining element.
>>> ' :: '.join(chickens)
'hen :: egg :: rooster'
>>> ' 🌿 '.join(chickens)
'hen 🌿 egg 🌿 rooster'
# Any iterable can be used as input.
>>> flowers = ("rose", "daisy", "carnation") # Tuples are iterable.
>>> '*-*'.join(flowers)
'rose*-*daisy*-*carnation'
>>> flowers = {"rose", "daisy", "carnation"} # Sets are iterable, but output order is not guaranteed.
>>> '*-*'.join(flowers)
'rose*-*carnation*-*daisy'
>>> phrase = "This is my string" # Strings are iterable, but be careful!
>>> '..'.join(phrase)
'T..h..i..s.. ..i..s.. ..m..y.. ..s..t..r..i..n..g'
# Separators are inserted **between** elements, but can be any string (including spaces).
# This can be exploited for interesting effects.
>>> under_words = ['under', 'current', 'sea', 'pin', 'dog', 'lay']
>>> separator = ' ⤴️ under'
>>> separator.join(under_words)
'under ⤴️ undercurrent ⤴️ undersea ⤴️ underpin ⤴️ underdog ⤴️ underlay'
# The separator can be composed different ways, as long as the result is a string.
>>> upper_words = ['upper', 'crust', 'case', 'classmen', 'most', 'cut']
>>> separator = ' 🌟 ' + upper_words[0]
>>> separator.join(upper_words)
'upper 🌟 uppercrust 🌟 uppercase 🌟 upperclassmen 🌟 uppermost 🌟 uppercut'
str中的码点可以从左侧通过0-based index来引用：
creative = '창의적인'
>>> creative[0]
'창'
>>> creative[2]
'적'
>>> creative[3]
'인'
从右侧同样可以取下标，起始为-1-based index：
creative = '창의적인'
>>> creative[-4]
'창'
>>> creative[-2]
'적'
>>> creative[-1]
'인'
Python 中没有单独的“字符”或“rune”类型，所以对字符串取下标会得到一个新的、长度为 1 的str：
>>> website = "exercism"
>>> type(website[0])
<class 'str'>
>>> len(website[0])
1
>>> website[0] == website[0:1] == 'e'
True
可以通过切片表示法选取子字符串，使用<str>[<start>:stop:<step>]来生成新字符串。
结果不包含stop下标。
如果未给出start，起始下标就是 0。
如果未给出stop，stop下标就是字符串的末尾。
moon_and_stars = '🌟🌟🌙🌟🌟⭐'
sun_and_moon = '🌞🌙🌞🌙🌞🌙🌞🌙🌞'
>>> moon_and_stars[1:4]
'🌟🌙🌟'
>>> moon_and_stars[:3]
'🌟🌟🌙'
>>> moon_and_stars[3:]
'🌟🌟⭐'
>>> moon_and_stars[:-1]
'🌟🌟🌙🌟🌟'
>>> moon_and_stars[:-3]
'🌟🌟🌙'
>>> sun_and_moon[::2]
'🌞🌞🌞🌞🌞'
>>> sun_and_moon[:-2:2]
'🌞🌞🌞🌞'
>>> sun_and_moon[1:-1:2]
'🌙🌙🌙🌙'
还可以用<str>.split(<separator>)把字符串拆成更小的字符串，它会返回一个由子字符串组成的list。
如有需要，还可以对这个列表继续取下标或拆分。
不传任何参数调用<str>.split()时，会按空白字符拆分字符串。
>>> cat_ipsum = "Destroy house in 5 seconds mock the hooman."
>>> cat_ipsum.split()
...
['Destroy', 'house', 'in', '5', 'seconds', 'mock', 'the', 'hooman.']
>>> cat_ipsum.split()[-1]
'hooman.'
>>> cat_words = "feline, four-footed, ferocious, furry"
>>> cat_words.split(', ')
...
['feline', 'four-footed', 'ferocious', 'furry']
<str>.split()的分隔符可以不止一个字符。
拆分匹配时使用的是整个字符串。
>>> colors = """red,
orange,
green,
purple,
yellow"""
>>> colors.split(',\n')
['red', 'orange', 'green', 'purple', 'yellow']
字符串支持所有常见序列操作。
可以用for item in <str>在循环中逐个迭代各个码点。
可以用for index, item in enumerate(<str>)在循环中同时迭代下标_和_元素。
>>> exercise = 'လေ့ကျင့်'
# Note that there are more code points than perceived glyphs or characters
>>> for code_point in exercise:
... print(code_point)
...
လ
ေ
့
က
ျ
င
်
့
# Using enumerate will give both the value and index position of each element.
>>> for index, code_point in enumerate(exercise):
... print(index, ": ", code_point)
...
0 : လ
1 : ေ
2 : ့
3 : က
4 : ျ
5 : င
6 : ်
7 : ့
学习练习
