2.5. Type Str¶

2.5.1. Type Definition¶

str is a sequence

Listing 2.17. str Type Definition¶

data = ''
data = 'Jan Twardowski'

data = """First line
Second line
Third line"""
# 'First line\nSecond line\nThird line'

2.5.2. Type Casting¶

str() converts argument to str
print() before printing on the screen runs str() on its arguments

Listing 2.18. str() converts argument to str¶

str('Moon')                     # 'Moon'
str(1969)                       # '1969'
str(13.37)                      # '13.37'

2.5.3. Single and Double Quotes¶

" and ' works the same

Choose one and keep consistency in code

Python console prefers single quote (') character

It matters for doctest, which compares two outputs character by character

For multiline always use double quote characters to be consistent with the docstring convention PEP 257

Listing 2.19. Python console prefers single quote (')¶

data = 'My name is José Jiménez'

print(data)
# 'My name is José Jiménez'

Listing 2.20. Python console prefers single quote (')¶

data = "My name is José Jiménez"

print(data)
# 'My name is José Jiménez'

Listing 2.21. It's better to use double quotes, when text has apostrophes. This is the behavior of Python console.¶

data = 'My name\'s José Jiménez'

print(data)
# "My name's José Jiménez"

Listing 2.22. HTML and XML uses double quotes to enclose attribute values, hence it's better to use single quotes for the string.¶

data = '<a href="http://python.astrotech.io">Python and Machine Learning</a>'

print(data)
# '<a href="http://python.astrotech.io">Python and Machine Learning</a>'

Listing 2.23. For multiline always use double quote characters to be consistent with the docstring convention PEP 257¶

data = """My name's "José Jiménez""""
data = '''My name\'s "José Jiménez"'''

2.5.4. Docstring¶

For multiline always use double quote characters to be consistent with the docstring convention PEP 257

More information in Function Doctest

Listing 2.24. If assigned to variable, it serves as multiline str otherwise it's a docstring.¶

"""
We choose to go to the Moon!
We choose to go to the Moon in this decade and do the other things,
not because they are easy, but because they are hard;
because that goal will serve to organize and measure the best of our energies and skills,
because that challenge is one that we are willing to accept, one we are unwilling to postpone,
and one we intend to win, and the others, too.
"""

2.5.5. 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

print('\U0001F680')     # 🚀

2.5.6. Format String¶

String interpolation (variable substitution)

Since Python 3.6

Used for str concatenation

name = 'José Jiménez'

print(f'My name... {name}')
# My name... José Jiménez

firstname = 'José'
lastname = 'Jiménez'
result = f'My name... {firstname} {lastname}'

print(result)
# My name... José Jiménez

2.5.7. 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'

2.5.8. 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

'Moon'              # Unicode (in Python 3)
b'Moon'             # Bytes Literal

'Moon'.encode()     # b'Moon'
b'Moon'.decode()    # 'Moon'

2.5.9. Raw String¶

Escapes does not matters

Listing 2.25. In Regular Expressions¶

r'[a-z0-9]\n'

print(r'C:\Users\Admin\file.txt')
# C:\Users\Admin\file.txt

print('C:\\Users\\Admin\\file.txt')
# C:\Users\Admin\file.txt

print('C:\Users\Admin\file.txt')
# SyntaxError: (unicode error) 'unicodeescape'
#   codec can't decode bytes in position 2-3: truncated \UXXXXXXXX escape

Problem: \Users
after \U... python expects Unicode codepoint in hex
s is invalid hexadecimal character

2.5.10. Reading Input¶

input() returns str

Good practice: add space at the end of prompt

Good practice: always .strip() text from user input

Good practice: always sanitize values from user prompt

Listing 2.26. input() function argument is prompt text, which "invites" user to enter specific information. Note colon space (": ") at the end. Space is needed to separate user input from prompt.¶

name = input('What is your name: ')  # Jan Twardowski<ENTER>

print(name)     # 'Jan Twardowski'
type(name)      # <class 'str'>

Listing 2.27. input() always returns a str. To get numeric value type casting to int is needed.¶

age = input('What is your age: ')  # 42<ENTER>

print(age)      # '42'
type(age)       # <class 'str'>

age = int(age)
print(age)      # 42
type(age)       # <class 'int'>

Listing 2.28. Conversion to float handles decimals, which int does not support¶

age = input('What is your age: ')  # 42.5<ENTER>

age = int(age)      # ValueError: invalid literal for int() with base 10: '42.5'
age = float(age)    # 42.5

print(age)          # 42.5
type(age)           # <class 'int'>

Listing 2.29. Conversion to float cannot handle comma (',') as a decimal separator¶

age = input('What is your age: ')  # 42,5<ENTER>

age = int(age)      # ValueError: invalid literal for int() with base 10: '45,5'
age = float(age)    # ValueError: could not convert string to float: '45,5'

2.5.11. Length¶

len('hello')
# 5

2.5.12. Concatenation¶

Preferred string concatenation is using f-string formatting

'a' + 'b'
# 'ab'

'1' + '2'
# '12'

text1 = 'a'
text2 = 'b'

text1 + text2
# 'ab'

a = '1'
b = '2'

a + b
'12'

'-' * 10                # '----------'
'Beetlejuice' * 3       # 'BeetlejuiceBeetlejuiceBeetlejuice'
'Mua' + 'Ha' * 2        # 'MuaHaHa'

2.5.13. String Immutability¶

firstname = 'Jan'
lastname = 'Twardowski'

firstname + lastname
# JanTwardowski

firstname + ' ' + lastname
# Jan Twardowski

Listing 2.30. How many string are there in a memory?¶

firstname = 'Jan'
lastname = 'Twardowski'

firstname + lastname

Listing 2.31. How many string are there in a memory?¶

firstname = 'Jan'
lastname = 'Twardowski'
age = 42

'Hello ' + firstname + ' ' + lastname + ' ' + str(age) + '!'

Listing 2.32. How many string are there in a memory?¶

firstname = 'Jan'
lastname = 'Twardowski'
age = 42

f'Hello {firstname} {lastname} {age}!'

2.5.14. Assignments¶

2.5.14.1. Type String Input¶

Complexity level: easy
Lines of code to write: 3 lines
Estimated time of completion: 3 min
Solution: solution/type_str_input.py

English

Ask user to input text
Print number of characters

Polish

Poproś użytkownika o wprowadzenie tekstu
Wypisz liczbę znaków

2.5.14.2. Type String Emoticon¶

Complexity level: easy
Lines of code to write: 2 lines
Estimated time of completion: 3 min
Solution: solution/type_str_emoticon.py

English

Ask user to input name
Print hello NAME EMOTICON, where:
- NAME is a name read from user
- EMOTICON is Unicode Codepoint "U0001F642"

Polish

Poproś użytkownika o wprowadzenie imienia
Wypisz hello NAME EMOTICON, gdzie:
- NAME to imię wczytane od użytkownika
- EMOTICON to Unicode Codepoint "U0001F642"

The whys and wherefores

Variable declaration
Print formatting
Reading input data from user

2.5.14.3. Type String Quotes¶

Complexity level: easy
Lines of code to write: 3 lines
Estimated time of completion: 8 min
Solution: solution/type_str_quotes.py

English

Ask user to input name
To print use f-string formatting
Note, that second line starts with tab
Value NAME in double quotes is a name read from user
Mind the different quotes, apostrophes, tabs and newlines
Do not use neither space not enter - use \n and \t
Do not use string addition (str + str)
Compare result with "Output" section (see below)

Polish

Poproś użytkownika o wprowadzenie imienia
Do wypisania użyj f-string formatting
Zauważ, że druga linijka zaczyna się od tabulacji
Wartość NAME w podwójnych cudzysłowach to ciąg od użytkownika
Zwróć uwagę na znaki apostrofów, cudzysłowów, tabulacji i nowych linii
Nie używaj spacji ani entera - użyj \n i \t
Nie korzystaj z dodawania stringów (str + str)
Porównaj wyniki z sekcją "Output" (patrz poniżej)

Output

'''My name... "NAME".
    I'm an """astronaut!"""'''

The whys and wherefores

Variable declaration
Print formatting
Reading input data from user