Learning notes, memorandums

String type

Created as an instance of the str class. There is no distinction between characters and strings.

Basic method

Basic methods of string type

a = ' Hello, World \n'
 b = a.strip () # Remove whitespace before and after
 c = b.lower () # Convert to lowercase
 d = b.upper () # Convert to uppercase
 e = b.swapcase () # Invert case
 f = b.title () # Convert only the first letter of a word to uppercase
 g = b.split ('') # Split into list by specifying delimiter
 h = b.splitlines () # Split into a list separated by line breaks

Creating a character string considering memory efficiency

Be careful when the loop causes multiple additional string operations, such as when you want to read text data from a file and save that data as a str instance.

Bad example

text = ''

with open('test.txt') as f:
    temp = f.readline()
    while temp:
        text += temp
        temp = f.readline()

In this description method, for each loop, one line of memory is allocated, the memory used up to the previous one and the memory newly read are newly allocated, and the old memory used up to the previous one is released. Since the processing is performed, the memory efficiency is very low and the processing speed is also slow.

Correct example

list = []

with open('test.txt') as f:
    temp = f.readline()
    while temp:
        list.append(temp)
        temp = f.readline()

text = ``.join(list)

To avoid compromising memory efficiency, you can save it as a list and combine it later. By using the join method, you can join the contents of the list by specifying the delimiter character.

Search for strings

startswith (): Whether the string starts with the string specified by the argument ʻEndswith () `: Whether the string ends with the string specified by the argument

find (): Get the index of the character string specified by the argument, -1 if not found ʻIndex (): Get the index of the character string specified by the argument, ValueError if not found You can also use the ʻin operator to determine if it is included.

count (): Count the number of times the character string specified by the argument appears.

String replacement

replace (): Replaces the character string specified by the first argument with the character string specified by the second argument, even if there are multiple target character strings, all are replaced.

Regular expressions

Methods that exist in the re package
re.match (): Returns the match () class if the beginning of the string matches the string specified by the argument, and None if it does not match. You can get the matched location with re.match (). span ().
re.search (): Returns the match () class if it matches the string specified by the argument, and None if it does not match. You can get the matched location with re.search (). span ().
If you use regular expressions repeatedly, you should compile them in advance using the compile () method.
findall (): Get all matching strings and save as a list.

+ Represents one or more repetitions, and * represents zero or more repetitions.

Replacement by external variable

By prefixing the string with f or F, the part enclosed by {} becomes a field that is replaced by an external variable or calculation formula.

a = 'World'
b = f'Hello, {a}'
print(b)

c = 10
d = 20
e = F'{c} + {d} = {c+d}'
print(e)

Execution result

Hello, World 10 + 20 = 30

Format specification

The format () method replaces the index or variable in the {} field with an argument.

a = '{0} is {1}'
b = a.format('apple', 'red')
print(b)

c = '{fruits} is {color}'
d = c.format(fruits = 'melon', color = 'green')
print(d)