Python Language String

Internal

• Python Language
• Printing to stdout in Python
• Go Strings

TODO

• PyOOP "Strings and Serialization"
• PyOOP "Strings"
• PyOOP "String manipulation"
• PyOOP "String formatting"
• PyOOP "Escaping braces"
• PyOOP "f-strings can contain Python code"
• PyOOP "Making it look right"
• PyOOP "Custom formatters"
• PyOOP "The format method"
• PyOOP "Strings are Unicode"
• PyOOP "Converting bytes to text"
• PyOOP "Converting text to bytes"
• PyOOP "Mutable byte strings"

Overview

String are a Python sequence of characters. Strings are immutable, a character within a string cannot be changed in-place once the string object has been instantiated. Python 3 supports the Unicode standard, so Python 3 strings can contain characters from any written language in the world.

Strings are sequences of Unicode characters and therefore can be treaded like other sequence like lists and tuples: they can be sliced or iterated upon.

Strings are Immutable

A string cannot be modified with the assignment operator:

s = "something"
s[3] = "s"

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[10], line 1
----> 1 s[3] = "s"

TypeError: 'str' object does not support item assignment

Declaring Strings

Quotes

String literals can be declared using four type of quotes: sigle quotes '...', double quotes "...", triple single quotes '''...''' and triple double quotes """...""".

Single and Double Quotes

s1 = 'abc'
s2 = "xyz"

Declaring string literals bounded by single and double quotes is equivalent. There are two types of quotes to make it possible to create strings that include single and double quotes: a single-quoted string allows specifying double quotes inside and a double-quoted string allows specifying single quotes inside:

s1 = 'the color is "red"'
s2 = "the shape is 'square'"

Three Single and Three Double Quotes

Multi-line string literals can be declared using three single quotes or three double quotes. The leading and training space in such strings will also be preserved. The attempt to declare multi-line string literals bounded by single or double quotes results in SyntaxError exceptions.

s1 = '''
    the color
        is
    "red"
    '''
s2 = """
    the shape
        is
    'square'
    """
print(s1)
print(s2)

The result is:

    the color
        is
    "red"


    the shape
        is
    'square'

F-String (Literal String Interpolation)

PEP 498

An f-string, short for formatted string literal is a literal string, prefixed with "f", which contains expressions inside curly braces. The expressions are replaced with their values. Introduced by PEP 498. Any kind of string (single-quote enclosed, double-quote enclosed and triple-quote enclosed) can be an f-string.

name = "long"
print(f'my name is {name}')
print(f"my name is {name}")
print(f"""

   my name is {name}

""")

Format specifiers can be added after each expression:

amount = 10.10
currency = "Euros"
usd = 9
s = f"{amount:0.2f} {currency:s} are worth USD {usd:d}"
print(s) # 10.10 Euros are worth USD 9

Indent to the right over 6 spaces:

i = 10
print(f"{i:>6}")

Escaped Characters

Python allows escaping the meaning of some characters by preceding the character with a backslash (\). Commonly escaped characters:

• New line: \n, which allows creating a multi-line string from a one-line string.
• Tab: \t
• Backslash itself: \\
• Single quote \' to introduce single quotes in single-quoted strings.
• Double quote \" to introduce double quotes in double-quoted strings.

print() resolves the escaped characters before sending them to stdout.

Empty String

An empty string can be declared using all kinds of quotes described above:

s1 = ''
s2 = ""
s3 = ''''''
s4 = """"""

Empty strings are evaluated to False in conditional expressions and can be tested with if:

s = ''
if s:
  print("NOT empty")
else:
  print("empty")

if not s:
  print("empty")

Emptiness can also be tested with:

s = ''
if len(s) != 0:
  print("NOT empty")
else:
  print("empty")

A string that contains blank space is not considered empty.

String type()

The function type() applied to a list returns:

<class 'str'>

To check whether an instance is a string:

i = ...
if type(i) is str:
  ...

Type Conversions from and to String

Convert other Data Types to Strings with str()

Other data types can be converted to string using the str() function. Python uses the str() function internally when print() is called on objects that are not strings, and when doing string interpolation.

Check whether a String can be Converted to an int

Use isdigit() string methods.

s = "something"
s2 = "10"
assert s.isdigit() is not True
assert s2.isdigit() is True

Convert a String to List

s = "abc"
l = list(s)
print(l) # ['a', 'b', 'c']

Also see:

list()

String Equality

TODO

s1 = "abc"
s2 = "abc"
s3 = "xyz"
assert s1 == s2
assert s2 != s3

Reading Strings

https://www.pythonforbeginners.com/basics/string-manipulation-in-python

String Length

The length of the string is returned by the built-in len() function.

s = 'abc'
assert len(s) == 3

The [] Operator and String Slices

The [...:...] syntax is called slicing and it implemented for many kinds of Python sequences.

Reading Individual Characters

The [] operator takes an offset from the beginning of the string, going from left to right (positive) or from the first position after the end of the string, going from right to left (negative) and returns the character.

s = 'abc'
assert s[0] == 'a'
assert s[1] == 'b'
assert s[-1] == 'c'
assert s[-2] == 'b'

The [] operator can be used to read strings from the sequence, but not modify the sequence. Because strings are immutable, an attempt to change a character at a specific position in string will throw an TypeError exception:

s = 'abc'
s[0] = 'x'
[...]
TypeError: 'str' object does not support item assignment

If the offset reaches beyond the string boundaries, applying the [] operator results in an IndexError: string index out of range exception.

Get the First Character of a String

s = "something"
assert s[0] == 's'

Get the Last Character of a String

s = "something"
assert s[-1] == 'g'

Reading Substrings with the Slice Operator [start:end:step]

The slice operator [start:end:step] extracts a substring defined as follows:

start

start specifies the offset of the first character, from the beginning of the string. If it's missing, 0 is implied. Unlike the [] operator, if the start offset falls beyond the edge of the string, no exception is thrown, but the empty string is returned instead: a value beyond of the end of the string is assumed to be the end of the string.

s = 'blue'
assert s[:] == 'blue'
assert s[0:] == 'blue'
assert s[2:] == 'ue'
assert s[4:] == ''
assert s[5:] == ''

Negative values can be used, which mean an offset from the end of the string, instead of the beginning.

s = 'blue'
assert s[-1:] == 'e'

end

end represents the offset of the first character that is not included in the substring. If it is not specified, the end is implied to be the first position outside of the string, so the whole end of the string is included. If the end offset falls outside the string, the whole end of the string is included.

s = 'blue'
assert s[:0] == ''
assert s[:1] == 'b'
assert s[:2] == 'bl'
assert s[:100] == 'blue'
# missing 'end' means the whole end of the string
assert s[2:] == 'ue'

Negative values can be used, which mean an offset from the beginning of the string, instead of the end.

s = 'blue'
assert s[:-2] == 'bl'

step

step means select every step character, starting from the start offset. If not specified, the default is 1.

s = 'blue'
assert s[::1] == 'blue'
assert s[::2] == 'bu'
assert s[::3] == 'be'

A negative step value means it steps backward, starting from the rightmost character of the slice. This is how a string is rendered backwards:

s = 'blue'
assert s[-1::-1] == 'eulb'

Get First n Characters as a String

s[0:n]
s[:n] # equivalent

Iterate over Characters of a String

s = "something"
for c in s:
  print(c)

s = "something"
for i in range(0, len(s)):
  print(s[i])

s = "something"
for i, c in enumerate(s):
  print(i, c)

String Manipulation and Processing

Combine Strings with the + Operator

Strings can be concatenated with the + operator:

'a' + 'b'

To concatenate strings and numbers, use type conversion function str():

'a' + str(1)

String literals (not variables) can be combined by just typing them one after another:

s = 'this is ' 'a ' 'string'
print(s)

Repeat a String a number of Times: Duplicate with the * Operator

The * operator can be used to be duplicate a string:

s = 'Ho ' * 3
print(s, 'said Santa')

It the multiplication factor is 0, the result is the empty string:

assert '' == 'something ' * 0

Split a String

<string-to-be-split>.split(<separator>)

s = 'A, B, C'
l = s.split(', ')
assert l[0] == 'A'
assert l[1] == 'B'
assert l[2] == 'C'

If no separator is specified, split() uses any sequence of white space characters (newline, space and tabs).

Join a String

<separator>.join(<list_of_strings>)

s = '@'.join(['test', 'example.com'])
assert 'test@example.com' == s

Search and String Statistics Functions

The in Operator

s = 'blue is a color'
if 'is a' in s:
    print('found')

startswith()

assert 'something'.startswith('some')

endswith()

assert 'something'.endswith('thing')

find()

Finds the offset of the first occurrence of the given argument, by searching from the beginning of the string:

assert 'something'.find('ome') == 1

If the string is not found, the method returns -1.

rfind()

Finds the offset of the first occurrence of the given argument, starting from the back of the string:

assert 'elemental'.rfind('e') == 4

If the string is not found, the method returns -1.

count()

Counts the string occurrence:

assert 'We hope you had a pleasant trip from Seattle to San Francisco, and we want to hear about your experience traveling with United'.count('you') == 2

String Manipulation Functions

"Trim" a String with strip()

Remove the specified sequence from both ends of a string. Note that if the sequence occurs repeatedly, all occurrences will be stripped.

assert '....test..'.strip('.') == 'test'

If no arguments are given, whitespace is assumed:

assert '  \n   test \t\t     '.strip() == 'test'

The function can be used to remove extension form the name of a file:

assert 'SomeFile.py'.strip('.py') == 'SomeFile'

Capitalization capitalize(), title(), upper(), lower(), swapcase()

capitalize() capitalizes the first word.

title() capitalizes all words.

isupper() method returns True if all the characters are in upper case, otherwise False. Numbers, symbols and spaces are not checked, only alphabet characters.

upper() converts all characters to uppercase.

islower() method returns True if all the characters are in lower case, otherwise False. Numbers, symbols and spaces are not checked, only alphabet characters.

lower() converts all characters to lowercase.

Padding and Alignment with ljust(), rjust(), center()

Leading Padding

Leading padding can be done with rjust(). By default, the padding is done with spaces, but a specific character can be provided.

s = 'abc'
assert '  abc' == s.rjust(5)
assert '__abc' == s.rjust(5, '_')

To fill a string with zeroes to the left use zfill():

assert '004' == '4'.zfill(3)

If the argument is a number, the same effect can be achieved with:

n = 4
assert '004' == f'{n:03}'

Not really, review this: where the first character following : specifies the padding character, and the second is the total length of the string.

Trailing Padding

Trailing padding can be done with ljust(). By default, the padding is done with spaces, but a specific character can be provided.

s = 'abc'
assert 'abc  ' == s.ljust(5)
assert 'abc++' == s.ljust(5, '+')

Equivalent method:

s = 'abc'
assert 'abc' == f'{s:2}'
assert 'abc' == f'{s:3}'
assert 'abc ' == f'{s:4}'

The same pattern can be used to produce a whitespace string of a certain length:

s = ' '
assert '   ' == f'{s:3}'

Centering

A string can also be centered, by default between spaces or a specified character.

s = 'abc'
assert ' abc ' == s.center(5)
assert '--abc--' == s.center(7, '-')

replace() Function

The replace() function performs string substitution, creating a new string whose fragments matching the first argument are replaced with the second argument. It has an optional count argument. If the count argument is omitted, all occurrences are replaced.

s = 'tattle tale'
assert 'bittle tale' == s.replace('ta', 'bi', 1)
assert 'bittle bile' == s.replace('ta', 'bi')

To replace a regular expression, use:

Python Regular Expressions | Replacing Regular Expression Occurrences

Regular Expressions

Regular Expressions

Strings and Unicode

Unicode strings can be converted to the UTF-8 bytes representation using the encode() method, and back from UTF-8 bytes to unicode with decode():

s = "español"
s2 = s.encode("utf-8") 
print(s2) # prints b'espa\xc3\xb1ol'
s3 = s2.decode('utf-8')
print(s3) # prints español

Code Examples

Reading a Multi-Line String Line by Line

TODO: not complete yet, prints two extra blank lines:

mls = """
a
b
c
"""

for line in mls.split('\n'):
    print(line)

Reading a File Line by Line

TO PROCESS: https://www.geeksforgeeks.org/read-a-file-line-by-line-in-python/

Indent a Multiline String

s = "a\n  b\n  c\n"
indent = 6
s2 = '\n'.join([''.rjust(indent) + i for i in s.split('\n')])