Python 101

Task 1 - Read Textbook

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Watch Welcome to Python at the CLI video
Read chapter 2

Practice what you read
Watch The Video Lesson
Chapter 2 Quiz

Read chapter 3

Practice what you read
Watch The Video Lesson

Read chapter 4

Task 2 - Follow these coding exercises

Please try the following code examples yourself by opening up the Command Prompt (if using Windows) or Terminal (if using Linux/Mac), type in "python" and press Enter. You should see ">>>", if you don't then make sure you have Python installed.

Using Python as a Calculator

The interpreter acts as a simple calculator: you can type an expression at it and it will write the value. Expression syntax is straightforward: the operators +, -, * and / work just like in most other languages (for example, Pascal or C); parentheses (()) can be used for grouping. For example:

    >>> 2 + 2
    4
    >>> 50 - 5*6
    20
    >>> (50 - 5*6) / 4
    5.0
    >>> 8 / 5  # division always returns a floating point number
    1.6

The integer numbers (e.g. 2, 4, 20) have type int, the ones with a fractional part (e.g. 5.0, 1.6) have type float. Division (/) always returns a float. To do floor division and get an integer result (discarding any fractional result) you can use the // operator; to calculate the remainder you can use %:

    >>> 17 / 3  # classic division returns a float
    5.666666666666667
    >>>
    >>> 17 // 3  # floor division discards the fractional part
    5
    >>> 17 % 3  # the % operator returns the remainder of the division
    2
    >>> 5 * 3 + 2  # result * divisor + remainder
    17

    With Python, it is possible to use the ** operator to calculate powers
    >>>
    >>> 5 ** 2  # 5 squared
    25
    >>> 2 ** 7  # 2 to the power of 7
    128

The equal sign (=) is used to assign a value to a variable. Afterwards, no result is displayed before the next interactive prompt: There is full support for floating point; operators with mixed type operands convert the integer operand to floating point:

    >>> width = 20
    >>> height = 5 * 9
    >>> width * height
    900
    >>> 4 * 3.75 - 1
    14.0

Python String

Besides numbers, Python can also manipulate strings, which can be expressed in several ways. They can be enclosed in single quotes ('...') or double quotes ("...") with the same result. \ can be used to escape quotes:

    >>> 'spam eggs'  # single quotes
    'spam eggs'
    >>> 'doesn\'t'  # use \' to escape the single quote...
    "doesn't"
    >>> "doesn't"  # ...or use double quotes instead
    "doesn't"
    >>> '"Yes," he said.'
    '"Yes," he said.'
    >>> "\"Yes,\" he said."
    '"Yes," he said.'
    >>> '"Isn\'t," she said.'
    '"Isn\'t," she said.'

If you want to concatenate variables or a variable and a literal, use +:

    >>> prefix = 'Py'
    >>> prefix + 'thon123'
    'Python123'

Strings can be indexed (subscripted), with the first character having index 0. There is no separate character type; a character is simply a string of size one:

    >>> word = 'Python'
    >>> word[0]  # character in position 0
    'P'
    >>> word[5]  # character in position 5
    'n'

Indices may also be negative numbers, to start counting from the right:

    >>> word[-1]  # last character
    'n'
    >>> word[-2]  # second-last character
    'o'
    >>> word[-6]
    'P'

In addition to indexing, slicing is also supported. While indexing is used to obtain individual characters, slicing allows you to obtain substring:

    >>> word[0:2]  # characters from position 0 (included) to 2 (excluded)
    'Py'
    >>> word[2:5]  # characters from position 2 (included) to 5 (excluded)
    'tho'

Note how the start is always included, and the end always excluded. This makes sure that s[:i] + s[i:] is always equal to s:

    >>> word[:2] + word[2:]
    'Python'
    >>> word[:4] + word[4:]
    'Python'

Slice indices have useful defaults; an omitted first index defaults to zero, an omitted second index defaults to the size of the string being sliced.

    >>> word[:2]   # character from the beginning to position 2 (excluded)
    'Py'
    >>> word[4:]   # characters from position 4 (included) to the end
    'on'
    >>> word[-2:]  # characters from the second-last (included) to the end
    'on'

Python List

Python knows a number of compound data types, used to group together other values. The most versatile is the list, which can be written as a list of comma-separated values (items) between square brackets. Lists might contain items of different types, but usually the items all have the same type.

    >>> squares = [1, 4, 9, 16, 25]
    >>> squares
    [1, 4, 9, 16, 25]

Like strings (and all other built-in sequence type), lists can be indexed and sliced:

    >>> squares[0]  # indexing returns the item
    1
    >>> squares[-1]
    25
    >>> squares[-3:]  # slicing returns a new list
    [9, 16, 25]

Lists also support operations like concatenation:

    >>> squares + [36, 49, 64, 81, 100]
    [1, 4, 9, 16, 25, 36, 49, 64, 81, 100]

Unlike strings, which are immutable, lists are a mutable type, i.e. it is possible to change their content:

    >>> cubes = [1, 8, 27, 65, 125]  # something's wrong here
    >>> 4 ** 3  # the cube of 4 is 64, not 65!
    64
    >>> cubes[3] = 64  # replace the wrong value
    >>> cubes
    [1, 8, 27, 64, 125]

You can also add new items at the end of the list, by using the append() method (we will see more about methods later):

    >>> cubes.append(216)  # add the cube of 6
    >>> cubes.append(7 ** 3)  # and the cube of 7
    >>> cubes
    [1, 8, 27, 64, 125, 216, 343]

Assignment to slices is also possible, and this can even change the size of the list or clear it entirely:

    >>> letters = ['a', 'b', 'c', 'd', 'e', 'f', 'g']
    >>> letters
    ['a', 'b', 'c', 'd', 'e', 'f', 'g']
    >>> # replace some values
    >>> letters[2:5] = ['C', 'D', 'E']
    >>> letters
    ['a', 'b', 'C', 'D', 'E', 'f', 'g']
    >>> # now remove them
    >>> letters[2:5] = []
    >>> letters
    ['a', 'b', 'f', 'g']
    >>> # clear the list by replacing all the elements with an empty list
    >>> letters[:] = []
    >>> letters
    []

The built-in function len() also applies to lists:

    >>> letters = ['a', 'b', 'c', 'd']
    >>> len(letters)
    4

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