第二章Python基础
第二章Python基础
- 2.1 上节拾遗
- 2.2 二进制
- 2.3 字符编码
- 2.4 基本数据类型——数字
- 2.5 基本数据类型——字符串
- 2.6 基本数据类型——列表
- 2.7 基本数据类型——元组
- 2.8 可变、不可变数据类型和hash
- 2.9 基本数据类型——字典
- 2.10 基本数据类型——集合
- 2.11 collections模块
- 2.12 本章小结
2.1上节拾遗
例1:name = 'oldboy'
首先，当我们定义了一个变量name = ‘oldboy’的时候，在内存中其实是做了这样一件事：
程序开辟了一块内存空间，将‘oldboy’存储进去，再让变量名name指向‘oldboy’所在的内存地址。如下图所示：
例2:两个变量名一个值
提问：当我执行下面这段代码的时候，程序是怎么处理的呢？
name1 = 'oldboy'
name2 = 'oldboy'
我们猜想会有两种可能:
第一种情况:程序分别在内存中开辟了两块儿空间来存储‘oldboy’这个值，并且让name1和name2指向这两个值。如下左图
第二种情况:由于两个值内容一致，所以程序只开辟一块儿空间存储‘oldboy’，并让name1和name2只想着个值。如下右图
提问：大家来猜测一下会是哪种情况？
其实上面的两种猜想都是对的。正常情况下字符串在内存里就是如我们猜想的第一种情况一样，每一次创建一个变量都会在内存中申请一块儿空间。
但是，python认为一些“看起来像python标识符的字符”和小整数字在开发中是常用的，因此出于节省内存的角度思考，对于这部分字符串和数字做出了优化[-5,257)，python解释器会由于要定义的新变量内容与之前定义过的变量内容相同而不让这部分内容占用新的内存空间。
python为我们提供了一个id()方法，可以查看一个变量的内存地址。
>>> name1 = 'oldboy'
>>> name2 = 'oldboy'
>>> name1_id = id(name1)
>>> name2_id = id(name2)
>>> print(name1_id,name2_id)
(4459387232, 4459387232)
执行完这段代码就基本验证了我们的思想，由于‘oldboy’是一个简单的字符串，因此python解释器做了优化，内存里只有一个‘oldboy’，name1和name2都指向同一块儿内存地址。
如果是长字符串呢？就米有优化机制啦！
对于[-5,257)范围内的数字也有优化机制：
例3:一个变量名2个值
如下图所示：
例4:变量的赋值与修改
要想知道上面问题的结果是为什么，首先要了解在内存中两个变量的存储情况
身份运算
2.2二进制
二进制的定义：当前的计算机系统使用的基本上是二进制系统，数据在计算机中主要是以补码的形式存储的。计算机中的二进制则是一个非常微小的开关，用“开”来表示1，“关”来表示0。
进制与十进制转换
其实刚刚在无形中我们已经将10进制转换成2进制了，现在我们要再总结一遍。
刚才我们已经发现，二进制的第n位代表的十进制值都刚好遵循着2的n次方这个规律
填位大法：
先把他们代表的值依次写出来，然后再根据10进制的值把数填到相应位置，就好了～～～
十进制转二进制方法相同，只要对照二进制为1的那一位对应的十进制值相加就可以了。
2.3字符编码
文件存取编码转换图
常用编码介绍一览表
| 编码 | 制定时间 | 作用 | 所占字节数 |
|---|---|---|---|
| ASCII | 1967年 | 表示英语及西欧语言 | 8bit/1bytes |
| GB2312 | 1980年 | 国家简体中文字符集，兼容ASCII | 2bytes |
| Unicode | 1991年 | 国际标准组织统一标准字符集 | 2bytes |
| GBK | 1995年 | GB2312的扩展字符集，支持繁体字，兼容GB2312 | 2bytes |
| UTF-8 | 1992年 | 不定长编码 | 1-3bytes |
2.4基本数据类型——数字
布尔型
bool型只有两个值：True和False
之所以将bool值归类为数字，是因为我们也习惯用1表示True，0表示False。
整型
Python中的整数属于int类型，默认用十进制表示，此外也支持二进制，八进制,十六进制表示方式。
进制转换
尽管计算机只认识二进制，但是为了迎合我们的习惯，python中的数字默认还是十进制。还提供了一些方法来帮助我们做转换。比如是进制转换为二进制使用bin方法，在转换结果前面还会加上‘0b’表示是一个二进制书。
既然十进制可以转换为二进制，那么其实使用同样的原理也可以转换为其他进制，python也为我们提供了十进制转换成八进制和十六进制的方法，分别是oct和hex。八进制前面以‘0o’标示，十六进制以‘0x’标示
浮点型
浮点数是属于有理数中某特定子集的数的数字表示，在计算机中用以近似表示任意某个实数。具体的说，这个实数由一个整数或定点数（即尾数）乘以某个基数（计算机中通常是2）的整数次幂得到，这种表示方法类似于基数为10的科学计数法。
为什么要叫做float浮点型？
浮点数也就是小数，之所以称为浮点数，是因为按照科学记数法表示时，
一个浮点数的小数点位置是可变的，比如，
1.23*109和12.3*108是相等的。
浮点数可以用数学写法，如1.23，3.14，-9.01，等等。但是对于很大或很小的浮点数，就必须用科学计数法表示，把10用e替代：
1.23*109就是1.23e9，或者12.3e8，0.000012可以写成1.2e-5，等等。
整数和浮点数在计算机内部存储的方式是不同的，整数运算永远是精确的而浮点数运算则可能会有四舍五入的误差。
复数
从上面的图中我们就可以看出，复数complex是由实数和虚数组成的
要了解复数，其实关于复数还需要先了解虚数。虚数(就是虚假不实的数):平方为复数的数叫做虚数。
复数是指能写成如下形式的数a+bi，这里a和b是实数，i是虚数单位(即-1开根)。在复数a+bi中，a称为复数的实部，b称为复数的虚部(虚数是指平方为负数的数)，i称为虚数单位。
当虚部等于零时，这个复数就是实数；当虚部不等于零时，这个复数称为虚数。
注，虚数部分的字母j大小写都可以。
2.4基本数据类型——字符串
字符串的特性与常用操作
特性：
1.按照从左到右的顺序定义字符集合，下标从0开始顺序访问，有序
补充：
1.字符串的单引号和双引号都无法取消特殊字符的含义，如果想让引号内所有字符均取消特殊意义，在引号前面加r，如name＝r'l\thf'
2.unicode字符串与r连用必需在r前面，如name＝ur'l\thf'
常用操作：
#索引
s = 'hello'
>>> s[1]
'e'
>>> s[-1]
'o'
>>> s.index('e')
1
#查找
>>> s.find('e')
1
>>> s.find('i')
-1
#移除空白
s = ' hello,world! '
s.strip()
s.lstrip()
s.rstrip()
s2 = '***hello,world!***'
s2.strip('*')
#长度
>>> s = 'hello,world'
>>> len(s)
11
#替换
>>> s = 'hello world'
>>> s.replace('h','H')
'Hello world'
>>> s2 = 'hi，how are you？'
>>> s2.replace('h','H')
'Hi，How are you？'
#切片
>>> s = 'abcdefghigklmn'
>>> s[0:7]
'abcdefg'
>>> s[7:14]
'higklmn'
>>> s[:7]
'abcdefg'
>>> s[7:]
'higklmn'
>>> s[:]
'abcdefghigklmn'
>>> s[0:7:2]
'aceg'
>>> s[7:14:3]
'hkn'
>>> s[::2]
'acegikm'
>>> s[::-1]
'nmlkgihgfedcba'
字符串的工厂函数
教会学员看源码
class str(object):
"""
str(object='') -> str
str(bytes_or_buffer[, encoding[, errors]]) -> str
Create a new string object from the given object. If encoding or
errors is specified, then the object must expose a data buffer
that will be decoded using the given encoding and error handler.
Otherwise, returns the result of object.__str__() (if defined)
or repr(object).
encoding defaults to sys.getdefaultencoding().
errors defaults to 'strict'.
"""
def capitalize(self): # real signature unknown; restored from __doc__
"""
首字母变大写
S.capitalize() -> str
Return a capitalized version of S, i.e. make the first character
have upper case and the rest lower case.
"""
return ""
def casefold(self): # real signature unknown; restored from __doc__
"""
S.casefold() -> str
Return a version of S suitable for caseless comparisons.
"""
return ""
def center(self, width, fillchar=None): # real signature unknown; restored from __doc__
"""
原来字符居中，不够用空格补全
S.center(width[, fillchar]) -> str
Return S centered in a string of length width. Padding is
done using the specified fill character (default is a space)
"""
return ""
def count(self, sub, start=None, end=None): # real signature unknown; restored from __doc__
"""
从一个范围内的统计某str出现次数
S.count(sub[, start[, end]]) -> int
Return the number of non-overlapping occurrences of substring sub in
string S[start:end]. Optional arguments start and end are
interpreted as in slice notation.
"""
return 0
def encode(self, encoding='utf-8', errors='strict'): # real signature unknown; restored from __doc__
"""
encode(encoding='utf-8',errors='strict')
以encoding指定编码格式编码，如果出错默认报一个ValueError，除非errors指定的是
ignore或replace
S.encode(encoding='utf-8', errors='strict') -> bytes
Encode S using the codec registered for encoding. Default encoding
is 'utf-8'. errors may be given to set a different error
handling scheme. Default is 'strict' meaning that encoding errors raise
a UnicodeEncodeError. Other possible values are 'ignore', 'replace' and
'xmlcharrefreplace' as well as any other name registered with
codecs.register_error that can handle UnicodeEncodeErrors.
"""
return b""
def endswith(self, suffix, start=None, end=None): # real signature unknown; restored from __doc__
"""
S.endswith(suffix[, start[, end]]) -> bool
Return True if S ends with the specified suffix, False otherwise.
With optional start, test S beginning at that position.
With optional end, stop comparing S at that position.
suffix can also be a tuple of strings to try.
"""
return False
def expandtabs(self, tabsize=8): # real signature unknown; restored from __doc__
"""
将字符串中包含的\t转换成tabsize个空格
S.expandtabs(tabsize=8) -> str
Return a copy of S where all tab characters are expanded using spaces.
If tabsize is not given, a tab size of 8 characters is assumed.
"""
return ""
def find(self, sub, start=None, end=None): # real signature unknown; restored from __doc__
"""
S.find(sub[, start[, end]]) -> int
Return the lowest index in S where substring sub is found,
such that sub is contained within S[start:end]. Optional
arguments start and end are interpreted as in slice notation.
Return -1 on failure.
"""
return 0
def format(self, *args, **kwargs): # known special case of str.format
"""
格式化输出
三种形式：
形式一.
>>> print('{0}{1}{0}'.format('a','b'))
aba
形式二：（必须一一对应）
>>> print('{}{}{}'.format('a','b'))
Traceback (most recent call last):
File "<input>", line 1, in <module>
IndexError: tuple index out of range
>>> print('{}{}'.format('a','b'))
ab
形式三：
>>> print('{name} {age}'.format(age=12,name='lhf'))
lhf 12
S.format(*args, **kwargs) -> str
Return a formatted version of S, using substitutions from args and kwargs.
The substitutions are identified by braces ('{' and '}').
"""
pass
def format_map(self, mapping): # real signature unknown; restored from __doc__
"""
与format区别
'{name}'.format(**dict(name='alex'))
'{name}'.format_map(dict(name='alex'))
S.format_map(mapping) -> str
Return a formatted version of S, using substitutions from mapping.
The substitutions are identified by braces ('{' and '}').
"""
return ""
def index(self, sub, start=None, end=None): # real signature unknown; restored from __doc__
"""
S.index(sub[, start[, end]]) -> int
Like S.find() but raise ValueError when the substring is not found.
"""
return 0
def isalnum(self): # real signature unknown; restored from __doc__
"""
至少一个字符，且都是字母或数字才返回True
S.isalnum() -> bool
Return True if all characters in S are alphanumeric
and there is at least one character in S, False otherwise.
"""
return False
def isalpha(self): # real signature unknown; restored from __doc__
"""
至少一个字符，且都是字母才返回True
S.isalpha() -> bool
Return True if all characters in S are alphabetic
and there is at least one character in S, False otherwise.
"""
return False
def isdecimal(self): # real signature unknown; restored from __doc__
"""
S.isdecimal() -> bool
Return True if there are only decimal characters in S,
False otherwise.
"""
return False
def isdigit(self): # real signature unknown; restored from __doc__
"""
S.isdigit() -> bool
Return True if all characters in S are digits
and there is at least one character in S, False otherwise.
"""
return False
def isidentifier(self): # real signature unknown; restored from __doc__
"""
字符串为关键字返回True
S.isidentifier() -> bool
Return True if S is a valid identifier according
to the language definition.
Use keyword.iskeyword() to test for reserved identifiers
such as "def" and "class".
"""
return False
def islower(self): # real signature unknown; restored from __doc__
"""
至少一个字符，且都是小写字母才返回True
S.islower() -> bool
Return True if all cased characters in S are lowercase and there is
at least one cased character in S, False otherwise.
"""
return False
def isnumeric(self): # real signature unknown; restored from __doc__
"""
S.isnumeric() -> bool
Return True if there are only numeric characters in S,
False otherwise.
"""
return False
def isprintable(self): # real signature unknown; restored from __doc__
"""
S.isprintable() -> bool
Return True if all characters in S are considered
printable in repr() or S is empty, False otherwise.
"""
return False
def isspace(self): # real signature unknown; restored from __doc__
"""
至少一个字符，且都是空格才返回True
S.isspace() -> bool
Return True if all characters in S are whitespace
and there is at least one character in S, False otherwise.
"""
return False
def istitle(self): # real signature unknown; restored from __doc__
"""
>>> a='Hello'
>>> a.istitle()
True
>>> a='HellP'
>>> a.istitle()
False
S.istitle() -> bool
Return True if S is a titlecased string and there is at least one
character in S, i.e. upper- and titlecase characters may only
follow uncased characters and lowercase characters only cased ones.
Return False otherwise.
"""
return False
def isupper(self): # real signature unknown; restored from __doc__
"""
S.isupper() -> bool
Return True if all cased characters in S are uppercase and there is
at least one cased character in S, False otherwise.
"""
return False
def join(self, iterable): # real signature unknown; restored from __doc__
"""
#对序列进行操作（分别使用' '与':'作为分隔符）
>>> seq1 = ['hello','good','boy','doiido']
>>> print ' '.join(seq1)
hello good boy doiido
>>> print ':'.join(seq1)
hello:good:boy:doiido
#对字符串进行操作
>>> seq2 = "hello good boy doiido"
>>> print ':'.join(seq2)
h:e:l:l:o: :g:o:o:d: :b:o:y: :d:o:i:i:d:o
#对元组进行操作
>>> seq3 = ('hello','good','boy','doiido')
>>> print ':'.join(seq3)
hello:good:boy:doiido
#对字典进行操作
>>> seq4 = {'hello':1,'good':2,'boy':3,'doiido':4}
>>> print ':'.join(seq4)
boy:good:doiido:hello
#合并目录
>>> import os
>>> os.path.join('/hello/','good/boy/','doiido')
'/hello/good/boy/doiido'
S.join(iterable) -> str
Return a string which is the concatenation of the strings in the
iterable. The separator between elements is S.
"""
return ""
def ljust(self, width, fillchar=None): # real signature unknown; restored from __doc__
"""
S.ljust(width[, fillchar]) -> str
Return S left-justified in a Unicode string of length width. Padding is
done using the specified fill character (default is a space).
"""
return ""
def lower(self): # real signature unknown; restored from __doc__
"""
S.lower() -> str
Return a copy of the string S converted to lowercase.
"""
return ""
def lstrip(self, chars=None): # real signature unknown; restored from __doc__
"""
S.lstrip([chars]) -> str
Return a copy of the string S with leading whitespace removed.
If chars is given and not None, remove characters in chars instead.
"""
return ""
def maketrans(self, *args, **kwargs): # real signature unknown
"""
Return a translation table usable for str.translate().
If there is only one argument, it must be a dictionary mapping Unicode
ordinals (integers) or characters to Unicode ordinals, strings or None.
Character keys will be then converted to ordinals.
If there are two arguments, they must be strings of equal length, and
in the resulting dictionary, each character in x will be mapped to the
character at the same position in y. If there is a third argument, it
must be a string, whose characters will be mapped to None in the result.
"""
pass
def partition(self, sep): # real signature unknown; restored from __doc__
"""
以sep为分割，将S分成head,sep,tail三部分
S.partition(sep) -> (head, sep, tail)
Search for the separator sep in S, and return the part before it,
the separator itself, and the part after it. If the separator is not
found, return S and two empty strings.
"""
pass
def replace(self, old, new, count=None): # real signature unknown; restored from __doc__
"""
S.replace(old, new[, count]) -> str
Return a copy of S with all occurrences of substring
old replaced by new. If the optional argument count is
given, only the first count occurrences are replaced.
"""
return ""
def rfind(self, sub, start=None, end=None): # real signature unknown; restored from __doc__
"""
S.rfind(sub[, start[, end]]) -> int
Return the highest index in S where substring sub is found,
such that sub is contained within S[start:end]. Optional
arguments start and end are interpreted as in slice notation.
Return -1 on failure.
"""
return 0
def rindex(self, sub, start=None, end=None): # real signature unknown; restored from __doc__
"""
S.rindex(sub[, start[, end]]) -> int
Like S.rfind() but raise ValueError when the substring is not found.
"""
return 0
def rjust(self, width, fillchar=None): # real signature unknown; restored from __doc__
"""
S.rjust(width[, fillchar]) -> str
Return S right-justified in a string of length width. Padding is
done using the specified fill character (default is a space).
"""
return ""
def rpartition(self, sep): # real signature unknown; restored from __doc__
"""
S.rpartition(sep) -> (head, sep, tail)
Search for the separator sep in S, starting at the end of S, and return
the part before it, the separator itself, and the part after it. If the
separator is not found, return two empty strings and S.
"""
pass
def rsplit(self, sep=None, maxsplit=-1): # real signature unknown; restored from __doc__
"""
S.rsplit(sep=None, maxsplit=-1) -> list of strings
Return a list of the words in S, using sep as the
delimiter string, starting at the end of the string and
working to the front. If maxsplit is given, at most maxsplit
splits are done. If sep is not specified, any whitespace string
is a separator.
"""
return []
def rstrip(self, chars=None): # real signature unknown; restored from __doc__
"""
S.rstrip([chars]) -> str
Return a copy of the string S with trailing whitespace removed.
If chars is given and not None, remove characters in chars instead.
"""
return ""
def split(self, sep=None, maxsplit=-1): # real signature unknown; restored from __doc__
"""
以sep为分割，将S切分成列表，与partition的区别在于切分结果不包含sep，
如果一个字符串中包含多个sep那么maxsplit为最多切分成几部分
>>> a='a,b c\nd\te'
>>> a.split()
['a,b', 'c', 'd', 'e']
S.split(sep=None, maxsplit=-1) -> list of strings
Return a list of the words in S, using sep as the
delimiter string. If maxsplit is given, at most maxsplit
splits are done. If sep is not specified or is None, any
whitespace string is a separator and empty strings are
removed from the result.
"""
return []
def splitlines(self, keepends=None): # real signature unknown; restored from __doc__
"""
Python splitlines() 按照行('\r', '\r\n', \n')分隔，
返回一个包含各行作为元素的列表，如果参数 keepends 为 False，不包含换行符，如 果为 True，则保留换行符。
>>> x
'adsfasdf\nsadf\nasdf\nadf'
>>> x.splitlines()
['adsfasdf', 'sadf', 'asdf', 'adf']
>>> x.splitlines(True)
['adsfasdf\n', 'sadf\n', 'asdf\n', 'adf']
S.splitlines([keepends]) -> list of strings
Return a list of the lines in S, breaking at line boundaries.
Line breaks are not included in the resulting list unless keepends
is given and true.
"""
return []
def startswith(self, prefix, start=None, end=None): # real signature unknown; restored from __doc__
"""
S.startswith(prefix[, start[, end]]) -> bool
Return True if S starts with the specified prefix, False otherwise.
With optional start, test S beginning at that position.
With optional end, stop comparing S at that position.
prefix can also be a tuple of strings to try.
"""
return False
def strip(self, chars=None): # real signature unknown; restored from __doc__
"""
S.strip([chars]) -> str
Return a copy of the string S with leading and trailing
whitespace removed.
If chars is given and not None, remove characters in chars instead.
"""
return ""
def swapcase(self): # real signature unknown; restored from __doc__
"""
大小写反转
S.swapcase() -> str
Return a copy of S with uppercase characters converted to lowercase
and vice versa.
"""
return ""
def title(self): # real signature unknown; restored from __doc__
"""
S.title() -> str
Return a titlecased version of S, i.e. words start with title case
characters, all remaining cased characters have lower case.
"""
return ""
def translate(self, table): # real signature unknown; restored from __doc__
"""
table=str.maketrans('alex','big SB')
a='hello abc'
print(a.translate(table))
S.translate(table) -> str
Return a copy of the string S in which each character has been mapped
through the given translation table. The table must implement
lookup/indexing via __getitem__, for instance a dictionary or list,
mapping Unicode ordinals to Unicode ordinals, strings, or None. If
this operation raises LookupError, the character is left untouched.
Characters mapped to None are deleted.
"""
return ""
def upper(self): # real signature unknown; restored from __doc__
"""
S.upper() -> str
Return a copy of S converted to uppercase.
"""
return ""
def zfill(self, width): # real signature unknown; restored from __doc__
"""
原来字符右对齐，不够用0补齐
S.zfill(width) -> str
Pad a numeric string S with zeros on the left, to fill a field
of the specified width. The string S is never truncated.
"""
return ""
...略...
