Python learning (start and end) Chapter 1 variables, names and objects

Chapter 1

1.1 Variables, names and objects

• type(thing) Get the type of object

a = 12
type(a)
<class'int'>

• Characters and rules that variable names can contain
  • Lowercase letters (a~z)
  • Number (0~9)
  • Underscore (_)
  • The name is not allowed to start with a number
  • Python reserved keywords are not allowed

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1.2 Numbers

Operator	description	Example
+	addition	5+8=13
-	Subtraction	90-10=80
*	multiplication	4*7=28
/	Floating point division	7/2=3.5
//	Integer division	7//2=3
%	Modulus (remainder)	7%3=1
**	power	3**4=81

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1.2.1 Integer

• Operational equivalence

# +、-、*、/、//、**
a = 100
a = a - 3 等价 a -= 3

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1.2.2 Cardinality

• Base
  • 0b or 0B stands for binary
  • 0o or 0O stands for octal
  • 0x or 0X stands for hexadecimal

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1.3 String

• Use of quotation marks
  • Double quotation marks and single quotation marks nested—>make the string contain quotation marks
  • Triple quotes—>input and output multi-line strings
• Use \escaping
  • \nNew line
  • \t tab
  • \"Show quotation marks
  • \\Display single slash
• Splicing operation
  • Use + stitching
  • Use *copy
  • Use [] to extract characters

>>>letter = 'you are, you are'
>>>letter[-1]
'e'

• Use [start: end :step] to slice
  • [:]
  • [start:]
  • [:end] Extract from the beginning to end-1
  • [start:end] From the beginning to end-1
  • [start: end :step]
  • The offset starts from 0, 1 from left to right, and increases in sequence
  • From right to left, start from -1, -2 and decrease in turn
• Use len() to get the length
  • You can also use len() for other sequence types
• Use split() to split string.function(argumens)
  • The built-in string function split() can split a string into a list of several substrings based on the separator

>>>a = 'you are , you are beautiful.'
>>>a.split(',')
['you are','you are beautiful.']

• Use join() to merge string.join(list)
  • The join() function decomposes a list containing several substrings, and combines these substrings into a complete large string

>>>str_list = ['you are','you are beautiful.']
>>>str_list2 = ','.join(str_list)
>>>print(str_list2)
you are, you are beautiful.

• Commonly used string functions
  • a.startswith('all') —> Determine whether the string a starts with all
  • a.endswith('all') —> Determine whether a ends with all
  • a.find('all') —>Find the position (offset) where the word all first appears in a
  • a.rfind('all') —> the offset of the last occurrence of all
  • a.count('all') —> how many times all appears in a
  • a.isalnum() —> Determine whether all characters in a are letters or numbers
• Use replace() to replace
  • replace() performs simple substring replacement (the substring that needs to be replaced, the new substring used for replacement, and how many places need to be replaced) if the last parameter is omitted, only the first occurrence will be replaced by default

>>>setup.replace('duck','marmoset')
'a marmoset goes into a bar...'
#修改最多100处
>>>setup.repalce('a ','a famous',100)
'a famous duck goes into a famous bar...'

• Not commonly used string functions (case and alignment)

>>>setup = 'a duck goes into a bar...'
#将字符串收尾的.都删除
>>>setup.strip('.')
'a duck goes into a bar'
#让字符串首字母变成大写
>>>setup.capitalize()
'A duck goes into a bar...'
#让所有单词开头字母都变成大写
>>>setup.title()
'A Duck Goes Into A Bar...'
#让所有字母都转换为大写
>>>setup.upper()
'A DUCK GOES INTO A BAR...'
#将所有字母转换为小写
>>>setup.lower()
'a duck goes into a bar...'
#将所有字母的大小写转换
>>>setup.swapcase()
'a DUCK GOES INTO A BAR...'
#假设setup字符串被排版在指定长度（这里是30个字符）的空间里
#在30个字符位居中
>>>setup.center(30)
'  a duck goes into a bar...  '
#左对齐
>>>setup.ljust(30)
'a duck goes into a bar...  '
#右对齐
>>>setup.rjust(30)
'  a duck goes into a bar...'

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