2.1. Syntax Assignment

  • Identifier is a formal name for variable

  • Variables can change it's value during the program

  • In Python there are no constants

  • NameError when using not declared variable

  • AttributeError when cannot assign to variables

2.1.1. Variables

Identifiers are case sensitive. Use lowercase letters for variable names:

>>> name = 'Mark Watney'

By convention you should use use Latin characters and English names (Non-ASCII characters in an identifier):

>>> name = 'Mark'
>>> imię = 'Mark'

Underscore _ is used for multi-word names

>>> first_name = 'Mark'
>>> last_name = 'Watney'

You can also join words.

>>> firstname = 'Mark'
>>> lastname = 'Watney'

Although it works for two words, it could be hard to read for three or more:

You should always use lowercase letters:

>>> name = 'Mark Watney'
>>> Name = 'Mark Watney'

Capital letters by convention has different meaning. The code will run without errors or warnings, but you can mislead others. Remember code is read by 80% of a time, and written in 20%.

Not ok by convention :

>>> firstName = 'Mark'  # Camel Case - not used in Python
>>> Firstname = 'Mark'  # Pascal Case - reserved for class names
>>> FirstName = 'Mark'  # Pascal Case - reserved for class names

Camel Case convention is not used in Python. It is common in other programming language such as C / C++ / C# / Java / JavaScript.

You can put numbers in variables:

>>> name1 = 'Mark'
>>> name2 = 'Mark'

BUt the number cannot be the first character (otherwise will produce SyntaxError):

>>> 1name = 'Mark'  

2.1.2. Constants

Identifiers (variable/constant names) are case sensitive. Uppercase letters are used for constants (by convention):

>>> FILE = '/etc/passwd'
>>> FILENAME = '/etc/group'

Underscore _ is used for multi-word names:

>>> FILE_NAME = '/etc/shadow'

Python do not distinguish between variables and constants. Python allows you to change "constants" but it's a bad practice (good IDE will tell you):

>>> NAME = 'Mark Watney'
>>> NAME = 'Melissa Lewis'

2.1.3. Variables vs. Constants

  • Variables vs. constants - Names are case sensitive

>>> name = 'Mark Watney'  # variable
>>> NAME = 'Mark Watney'  # constant
>>> Name = 'Mark Watney'  # class

Definition of second, minute or hour does not change based on location or country (those values should be constants).

Definition of workday, workweek and workmonth differs based on location - each country can have different work times (those values should be variables).

>>> SECOND = 1
>>> MINUTE = 60 * SECOND
>>> HOUR = 60 * MINUTE
>>> workday = 8 * HOUR
>>> workweek = 40 * HOUR

For physical units it is ok to use proper cased names. It is better to be compliant with well known standard, than to enforce something which will mislead everyone.

>>> Pa = 1
>>> hPa = 100 * Pa
>>> kPa = 1000 * Pa
>>> MPa = 1000000 * Pa

The code above is far more readable, than the following snippet:

>>> PA = 1
>>> HPA = 100 * PA
>>> KPA = 1000 * PA
>>> MPA = 1000000 * PA

Note, that the only change was in variable names. As you can see, this could have a huge impact on describing the intent of what you want to achieve with the code.

2.1.4. Types

This concept is only briefly described here. More information will be in upcoming chapters:

Basic types:

>>> data = 1                 # int
>>> data = 1.2               # float
>>> data = True              # bool
>>> data = False             # bool
>>> data = None              # NoneType
>>> data = 'abc'             # str

Sequences:

>>> data = [1, 2, 3]         # list
>>> data = (1, 2, 3)         # tuple
>>> data = {1, 2, 3}         # set
>>> data = {'a': 1, 'b': 2}  # dict

2.1.5. Use Case - 0x01

>>> name = 'Mark Watney'
>>> age = 40
>>> height = 185.5
>>> is_astronaut = True
>>> friends = None

2.1.6. Assignments

"""
* Assignment: Syntax Variables Int
* Complexity: easy
* Lines of code: 1 lines
* Time: 2 min

English:
    1. Define variable `result` with value 1
    2. Run doctests - all must succeed

Polish:
    1. Zdefiniuj zmienną `result` z wartością 1
    2. Uruchom doctesty - wszystkie muszą się powieść

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

    >>> assert result is not Ellipsis, \
    'Assign your result to variable `result`'
    >>> assert type(result) is int, \
    'Variable `result` has invalid type, should be int'
    >>> assert result == 1, \
    'Variable `result` has invalid value, should be 1'
"""

# with value 1
# type: int
result = ...


"""
* Assignment: Syntax Variables Float
* Complexity: easy
* Lines of code: 1 lines
* Time: 2 min

English:
    1. Define variable `result` with value 1.2
    2. Run doctests - all must succeed

Polish:
    1. Zdefiniuj zmienną `result` z wartością 1.2
    2. Uruchom doctesty - wszystkie muszą się powieść

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

    >>> assert result is not Ellipsis, \
    'Assign your result to variable `result`'
    >>> assert type(result) is float, \
    'Variable `result` has invalid type, should be float'
    >>> assert result == 1.2, \
    'Variable `result` has invalid value, should be 1.2'
"""

# with value 1.2
# type: float
result = ...


"""
* Assignment: Syntax Variables Str
* Complexity: easy
* Lines of code: 1 lines
* Time: 2 min

English:
    1. Define `result` with text 'Hello World'
    2. Run doctests - all must succeed

Polish:
    1. Zdefiniuj zmienną `result` z tekstem 'Hello World'
    2. Uruchom doctesty - wszystkie muszą się powieść

Hints:
    * Either quotes (") or apostrophes (') will work

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

    >>> assert result is not Ellipsis, \
    'Assign your result to variable `result`'
    >>> assert type(result) is str, \
    'Variable `result` has invalid type, should be str'
    >>> assert result == 'Hello World', \
    'Variable `result` has invalid value, should be "Hello World"'
"""

# with Hello World
# type: str
result = ...

"""
* Assignment: Syntax Variables Bool
* Complexity: easy
* Lines of code: 1 lines
* Time: 2 min

English:
    1. Define variable `result` with value True
    2. Run doctests - all must succeed

Polish:
    1. Zdefiniuj zmienną `result` z wartością True
    2. Uruchom doctesty - wszystkie muszą się powieść

Hint:
    * Note that True is capitalized

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

    >>> assert result is not Ellipsis, \
    'Assign your result to variable `result`'
    >>> assert type(result) is bool, \
    'Variable `result` has invalid type, should be bool'
    >>> assert result is True, \
    'Variable `result` has invalid value, should be True'
"""

# with value True
# type: bool
result = ...


"""
* Assignment: Syntax Variables None
* Complexity: easy
* Lines of code: 1 lines
* Time: 2 min

English:
    1. Define variable `result` with value None
    2. Run doctests - all must succeed

Polish:
    1. Zdefiniuj zmienną `result` z wartością None
    2. Uruchom doctesty - wszystkie muszą się powieść

Hint:
    * Note that None is capitalized

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

    >>> assert result is not Ellipsis, \
    'Assign your result to variable `result`'
    >>> assert type(result) is type(None), \
    'Variable `result` has invalid type, should be None'
    >>> assert result is None, \
    'Variable `result` has invalid value, should be None'
"""

# With value None
# type: None
result = ...