Basic grammar of Python3 system (character string)

Overview

You will study the basic grammar of Python 3 by referring to "Introduction to Python 3" by O'Reilly Japan. I hope it will be helpful for those who want to study Python in the same way.

About Python strings

Since the string is immutable (the value cannot be changed later), the string cannot be rewritten. For example, when changing the first character of ABC to G, an error will occur if the following method is tried.

>>> target = 'ABC'
>>> target[0] = 'G'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'str' object does not support item assignment

If you want to rewrite it, use a function etc. to get the operation result as a new character string.

Creation / extraction of character strings

Creating a string

>>> #When using single quotes
>>> 'sample'
'sample'

>>> #When using double quotes
>>> "sample"
'sample'

>>> #Single quote(Three)
>>> '''sample'''
'sample'

>>> #Double quote(Three)
>>> """sample"""
'sample'

>>> #Empty string
>>> ''
''

Character concatenation

>>> #Concatenation of literal strings
>>> 'ABC' + 'DEF'
'ABCDEF'

>>> #In the case of literal strings, you can concatenate them just by arranging them.
>>> 'ABC' 'DEF'
'ABCDEF'

>>> #Concatenation of string variables
>>> target1 = 'ABC'
>>> target2 = 'DEF'
>>> target1 + target2
'ABCDEF'

>>> #In the case of string variables, just arranging them will result in an error
>>> target1 = 'ABC'
>>> target2 = 'DEF'
>>> target1 target2
  File "<stdin>", line 1
    target1 target2
                  ^
SyntaxError: invalid syntax

Repeat string

>>> 'pyon ' * 5
'pyon pyon pyon pyon pyon '

Extract a specific character from a string

>>> target = 'ABC'
>>> target[0]
'A'
>>> target[1]
'B'
>>> target[2]
'C'

slice

You can use slices to extract substrings from a string.

# [start:end:step]
#   start :Start offset
#   end   :Tail offset
#   step  :Step (every character)

>>> target = 'ABCDEFGHIJKLMN'

>>> #Get the entire string
>>> target[:]
'ABCDEFGHIJKLMN'

>>> #From offset 5 to the end
>>> target[5:]
'FGHIJKLMN'

>>> #Offset 2 to 5
>>> target[2:5]
'CDE'

>>> #Last 3 letters
>>> target[-3:]
'LMN'

>>> #Offset 0 to 10 every 2 steps
>>> target[0:10:2]
'ACEGI'
>>> #It can be omitted like this
>>> target[:10:2]
'ACEGI'

Operation by function

Type conversion by str ()

>>> str(123)
'123'
>>> str(True)
'True'

Replace with replace ()

>>> target = 'ABC'
>>> target.replace('A','G')
'GBC'

Getting the length with len ()

>>> target = 'ABCDE'
>>> len(target)
5

Split by split ()

>>> #Specify a comma for the separator
>>> target = 'AB,C,DE,F'
>>> target.split(',')
['AB', 'C', 'DE', 'F']

>>> #If no separator is specified, whitespace characters (newlines, spaces, tabs) are treated as separators.
>>> target = 'A B C DE F'
>>> target.split()
['A', 'B', 'C', 'DE', 'F']

Join with join ()

The join () function is the inverse of the split () function

>>> #Specify the delimiter, then specify the list
>>> str_list = ['AB', 'C', 'DE', 'F']
>>> target = ','.join(str_list)
>>> print(target)
AB,C,DE,F

First character judgment by startswith ()

>>> target = 'ABCDEFGHIJKLMN'
>>> target.startswith('ABC')
True
>>> target.startswith('BCD')
False

First character judgment by endswith ()

>>> target = 'ABCDEFGHIJKLMN'
>>> target.endswith('LMN')
True
>>> target.endswith('KLM')
False

Get offset with find ()

>>> #If found
>>> target = 'ABCDEFGHIJKLMN'
>>> target.find('D')
3
>>> target.find('DE')
3
>>> #If not found-1 is returned
>>> target.find('DG')
-1
>>> target.find('T')
-1

Counting the number of cases by count ()

>>> target = 'AABBBCAAABCCCCCC'
>>> target.count('A')
5
>>> target.count('B')
4
>>> target.count('C')
7
>>> target.count('AA')
2
>>> target.count('D')
0

Change uppercase and lowercase

Change only the first letter to uppercase with capitalize ()

>>> target = 'thank you for coming.'
>>> target.capitalize()
'Thank you for coming.'

Change all words to title case with title ()

>>> target = 'thank you for coming.'
>>> target.title()
'Thank You For Coming.'

Change all characters to uppercase with upper ()

>>> target = 'thank you for coming.'
>>> target.upper()
'THANK YOU FOR COMING.'

Change all characters to lowercase with lower ()

>>> target = 'THANK YOU FOR COMING.'
>>> target.lower()
'thank you for coming.'

Reverse case with swapcase ()

>>> target = 'AbcdEFg'
>>> target.swapcase()
'aBCDefG'

Character arrangement

Centered with center ()

>>> #Centered in space 40
>>> target = 'thank you for coming.'
>>> target.center(40)
'         thank you for coming.          '

Place at the left end with ljust ()

>>> #Leftmost arrangement in space 40
>>> target = 'thank you for coming.'
>>> target.ljust(40)
'thank you for coming.                   '

Place at the right end with rjust ()

>>> target = 'thank you for coming.'
>>> target.rjust(40)
'                   thank you for coming.'

If you want to know other string functions

Standard documentation for string functions http://bit.ly/py-docs-strings