#Python's strings

Strings can be considered as a list of characters, and they are a very common and feature-rich type.

We have already touched upon strings in the section Basic Syntax - Variables and Basic Types.

#Encoding and bytes

Computers store data in binary using electronic components, so characters must be mapped to binary values. This mapping is called encoding.
For example, the English letter A corresponds to the binary value 01000001 (decimal 65) in ASCII encoding.

The smallest storage unit in a computer is a byte, which has 8 bits.
In Python, there is a type called bytes that stores a sequence of bytes.

A bytes literal looks similar to a string but is prefixed with a b, such as b'hello world'.

Converting a string to bytes is called encoding, and converting bytes back to string is called decoding:

data: bytes = b'hello world'
print(data)

text: str = data.decode()   # decode
print(text)
print(text.encode())        # encode

Bytes may look like strings with a b prefix, but there are key differences:

• Bytes elements are bytes, while string elements are characters (one character may occupy multiple bytes)
• Bytes can store non-text data, such as images

text: str = '你好世界'
print(text)

data: bytes = text.encode()  # encode
print(data)

print(len(text), len(data))  # lengths differ
print(text[1], data[1])      # text[1] is a whole Chinese character '好', while data[1] is a byte of the character '你'

#Old-style string formatting

In programming, you often need to create strings based on variables. You can use the % operator for formatting, with the syntax:

"format string" % (value1, value2, ...)    # values are in a tuple

If there is only one value, you can omit the tuple:

"format string" % value

For example:

print("Pork price is %d yuan per jin" % 15)
print("%d jin of pork costs %d yuan" % (3, 3*15))

Here %d is a decimal integer placeholder that will be replaced by the corresponding value in decimal form. Common placeholders include:

• %%: literal percent sign
• %d: decimal integer
• %o: octal integer
• %x: hexadecimal integer (lowercase)
• %X: hexadecimal integer (uppercase)
• %f: floating-point number
• %s: string

This style is less common nowadays; see more details at printf-style String Formatting.

#The format() method

The format method is more flexible than %. It uses curly braces {} as placeholders and supports formatting within the braces, for example:

print("Name: {}, Age: {}".format("Jerry", 18))               # positional replacement
print("Name: {1}, Age: {0}".format(19, "Tom"))              # positional index
print("Name: {name}, Age: {age}".format(name="Tuffy", age=8))# named replacement

You can specify width:

# Print multiplication table
for x in range(1, 10):
    for y in range(1, 10):
        print(' {:2} '.format(x * y), end='')  # min width 2 chars
    print('')

Width can be a variable:

print("'{:{width}}'".format('txt', width=7))

You can specify alignment:

print("'{:<5}'".format('txt'))  # left-align with width 5
print("'{:>5}'".format('txt'))  # right-align with width 5
print("'{:^5}'".format('txt'))  # center with width 5

You can use indexing for dicts:

score_list: dict[str,int] = {
    'Tom': 88,
    'Jerry': 99,
    'Spike': 66
}

print("Scores: Tom:{0[Tom]} Jerry:{0[Jerry]} Spike:{0[Spike]}".format(score_list))

To keep n decimal places using format function:

print("Approximate value of pi is {}".format(format(3.1415926, '.2f')))  # two decimal places

See Format String Syntax.

#Formatted string literals (f-strings)

Formatted string literals use the syntax f'xxxx' or f"xxxx", with expressions inside {} evaluated:

score_list: dict[str,int] = {
    'Tom': 88,
    'Jerry': 99,
    'Spike': 66
}

print(f"Scores: Tom:{score_list['Tom']} Jerry:{score_list['Jerry']} Spike:{score_list['Spike']}")

#Raw strings

Raw string literals are prefixed with r'xxxx' or r"xxxx", where escape sequences are not processed, so \n is treated as two characters, \ and n:

print(r'hello \n world')

Raw strings are useful for regular expressions or other scenarios requiring many backslashes.

Regular expressions will be covered later.

#Multiline strings

Multiline strings use triple quotes (''' or """), for example:

print(''' 
## Multiline strings

Multiline strings use triple quotes (`'''` or `"""`).
''')

Multiline strings are also commonly used as multiline comments:

'''
Not assigned to a variable and not evaluated,
so acts as a comment.
'''

print("hello world")

Multiline strings can also be combined with prefixes b, f, or r for bytes, formatted strings, or raw strings.