3.6. String Literals

3.6.1. Escape Characters

  • \n - New line (ENTER)

  • \t - Horizontal Tab (TAB)

  • \' - Single quote ' (escape in single quoted strings)

  • \" - Double quote " (escape in double quoted strings)

  • \\ - Backslash \ (to indicate, that this is not escape char)

  • More information in Builtin Printing

  • https://en.wikipedia.org/wiki/List_of_Unicode_characters

>>> print('Hello\World')
Hello\World
>>> print('Hello\nWorld')
Hello
World
>>> print('Hello\tWorld')  
Hello   World

3.6.2. Unicode

>>> print('\U0001F680')
🚀
>>> a = '\U0001F9D1'  # 🧑
>>> b = '\U0000200D'  # ''
>>> c = '\U0001F680'  # 🚀
>>>
>>> astronaut = a + b + c
>>> print(astronaut)
🧑‍🚀

3.6.3. Format String

  • String interpolation (variable substitution)

  • Since Python 3.6

  • Used for str concatenation

>>> name = 'Mark'
>>>
>>> print('Hello {name}')
Hello {name}
>>> name = 'Mark'
>>> print(f'Hello {name}')
Hello Mark

3.6.4. Unicode Literal

  • In Python 3 str is Unicode

  • In Python 2 str is Bytes

  • In Python 3 u'...' is only for compatibility with Python 2

>>> u'zażółć gęślą jaźń'
'zażółć gęślą jaźń'

3.6.5. Bytes Literal

  • Used while reading from low level devices and drivers

  • Used in sockets and HTTP connections

  • bytes is a sequence of octets (integers between 0 and 255)

  • bytes.decode() conversion to unicode str

  • str.encode() conversion to bytes

>>> data = 'Moon'   # Unicode Literal
>>> data = u'Moon'  # Unicode Literal
>>> data = b'Moon'  # Bytes Literal

Encode string from unicode (UTF-8) string to bytes:

>>> data = 'cześć'
>>>
>>> data.encode()
b'cze\xc5\x9b\xc4\x87'

Decode string from bytes to unicode (UTF-8):

>>> data = b'cze\xc5\x9b\xc4\x87'
>>>
>>> data.decode()
'cześć'

Unicode (UTF-8) is a default encoding. You can also specify different encodings to encode and decode data:

>>> data = 'cześć'
>>>
>>>
>>> data.encode('utf-8')
b'cze\xc5\x9b\xc4\x87'
>>>
>>> data.encode('iso-8859-2')
b'cze\xb6\xe6'
>>>
>>> data.encode('windows-1250')
b'cze\x9c\xe6'
>>>
>>> data.encode('cp1250')
b'cze\x9c\xe6'

3.6.6. Raw String

  • Escapes does not matters

>>> print('Print "\n" to get new line')
Print "
" to get new line
>>> print('Print "\\n" to get new line')
Print "\n" to get new line

3.6.7. Use Case - 0x01

Raw-string in Regular Expressions:

>>> '\\b[a-z]+\\b'
'\\b[a-z]+\\b'
>>> r'\b[a-z]+\b'
'\\b[a-z]+\\b'

3.6.8. Use Case - 0x02

Raw-string in escaping tab character:

>>> print('C:\watney\temporary.txt')  
C:\watney       emporary.txt
>>>
>>> print(r'C:\watney\temporary.txt')
C:\watney\temporary.txt

Raw-string in escaping newline character:

>>> print('C:\nasa\myfile.txt')
C:
asa\myfile.txt
>>>
>>> print(r'C:\nasa\myfile.txt')
C:\nasa\myfile.txt

Raw-string in escaping newline and tab character:

>>> print('C:\nasa\temporary.txt')  
C:
asa     emporary.txt
>>>
>>> print(r'C:\nasa\temporary.txt')
C:\nasa\temporary.txt

3.6.9. Use Case - 0x03

There are no problems with escapes in POSIX compliant paths:

>>> path = '/home/mwatney/myfile.txt'  # Linux
>>> path = '/User/mwatney/myfile.txt'  # macOS

In Windows you can find escape character in paths. In order to avoid problems you can use slashes instead of backslashes:

>>> path = 'c:/Users/mwatney/myfile.txt'

This is not typical for this operating system, therefore hardly anyone does that. Typically users will put paths using slashes, and that's ok, if you are using escaped slashes or raw-strings:

>>> path = 'c:\\Users\\mwatney\\myfile.txt'
>>> path = r'c:\Users\mwatney\myfile.txt'

As soon as you forget about using either of them, the problem occurs:

>>> path = 'c:\Users\mwatney\myfile.txt'
Traceback (most recent call last):
SyntaxError: (unicode error) 'unicodeescape' codec can't decode bytes in position 2-3: truncated \UXXXXXXXX escape

Problem is with \Users. After escape sequence \U... Python expects hexadecimal Unicode codepoint, i.e. '\U0001F680' which is a rocket 🚀 emoticon. In this example, Python finds letter s, which is invalid hexadecimal character and therefore raises an SyntaxError telling user that there is an error with decoding bytes. The only valid hexadecimal numbers are 0123456789abcdefABCDEF and s isn't one of them.

3.6.10. Assignments

Code 3.22. Solution
"""
* Assignment: String Literals Emoticon
* Required: yes
* Complexity: easy
* Lines of code: 2 lines
* Time: 3 min

English:
    1. Print `Hello 😀`
    2. Run doctests - all must succeed

Polish:
    1. Wypisz `Hello 😀`
    2. Uruchom doctesty - wszystkie muszą się powieść

Hints:
    * 😀 unicode codepoint is `\U0001F600`

Tests:
    >>> import sys; sys.tracebacklimit = 0

    >>> assert result is not Ellipsis, \
    'Assign result to variable: `result`'
    >>> assert type(result) is str, \
    'Variable `result` has invalid type, should be str'

    >>> '😀' in result
    True
    >>> result
    'Hello 😀'
"""

# str: Hello 😀
result = ...