>>> 2 + 3 * 6
20
>>> (2 + 3) * 6
30
>>> 48565878 * 578453
28093077826734
>>> 2 ** 8
256
>>> 23 / 7
3.2857142857142856
>>> 23 // 7
3
>>> 23 % 7
2
>>> 2 + 2
4
>>> (5 - 1) * ((7 + 1) / (3 - 1))
16.0

| | | | |

In this course I expect all functions to be commented according to the following style guide. This style guide was not made up, and is not a "hoop" to jump through. This is based (with some modifications) on Google's Python Style Guide that they require that all of their developers follow.

Python dynamically generates help documentation based on the docstrings that you create.

Each function must start with a docstring that includes:

A one sentence description Optional: A paragraph expanding on the one sentence description (not needed for simple functions) List of arguments with their usage and type Return value, it's usage and type Assumptions and conditions Optional: Errors that can be thrown (We haven't covered error handling yet, so feel free to ignore this for now)

While the integer, floating-point, and string data types have an unlimited number of possible values, the Boolean data type has only two values: True and False. (Boolean is capitalized because the data type is named after mathematician George Boole.) When typed as Python code, the Boolean values True and False lack the quotes you place around strings, and they always start with a capital T or F, with the rest of the word in lowercase.

Comparison operators compare two values and evaluate down to a single Boolean value.

The three Boolean operators (and, or, and not) are used to compare Boolean values. Like comparison operators, they evaluate these expressions down to a Boolean value.

The and and or operators always take two Boolean values (or expressions), so they’re considered binary operators. The and operator evaluates an expression to True if both Boolean values are True; otherwise, it evaluates to False.

On the other hand, the or operator evaluates an expression to True if either of the two Boolean values is True. If both are False, it evaluates to False.

Unlike and and or, the not operator operates on only one Boolean value (or expression). The not operator simply evaluates to the opposite Boolean value.

Since the comparison operators evaluate to Boolean values, you can use them in expressions with the Boolean operators.

Recall that the and, or, and not operators are called Boolean operators because they always operate on the Boolean values True and False. While expressions like 4 < 5 aren’t Boolean values, they are expressions that evaluate down to Boolean values. Try entering some Boolean expressions that use comparison operators into the interactive shell.

The computer will evaluate the left expression first, and then it will evaluate the right expression. When it knows the Boolean value for each, it will then evaluate the whole expression down to one Boolean value. You can think of the computer’s evaluation process for (4 < 5) and (5 < 6)

The Boolean operators have an order of operations just like the math operators do. After any math and comparison operators evaluate, Python evaluates the not operators first, then the and operators, and then the or operators.

Flow control statements often start with a part called the condition, and all are followed by a block of code called the clause. Before you learn about Python’s specific flow control statements, I’ll cover what a condition and a block are.

which are the same thing as expressions; condition is just a more specific name in the context of flow control statements. Conditions always evaluate down to a Boolean value, True or False. A flow control statement decides what to do based on whether its condition is True or False, and almost every flow control statement uses a condition.

Lines of Python code can be grouped together in blocks. You can tell when a block begins and ends from the indentation of the lines of code. There are three rules for blocks.

1. Blocks begin when the indentation increases.
2. Blocks can contain other blocks.
3. Blocks end when the indentation decreases to zero or to a containing block’s indentation.

Blocks are easier to understand by looking at some indented code

The program execution (or simply, execution) is a term for the current instruction being executed. If you print the source code on paper and put your finger on each line as it is executed, you can think of your finger as the program execution.

Not all programs execute by simply going straight down, however. If you use your finger to trace through a program with flow control statements, you’ll likely find yourself jumping around the source code based on conditions, and you’ll probably skip entire clauses.

The most common type of flow control statement is the if statement. An if statement’s clause (that is, the block following the if statement) will execute if the statement’s condition is True. The clause is skipped if the condition is False.

In plain English, an if statement could be read as, “If this condition is true, execute the code in the clause.” In Python, an if statement consists of the following:

• The if keyword
• A condition (that is, an expression that evaluates to True or False)
• A colon
• Starting on the next line, an indented block of code (called the if clause)

An if clause can optionally be followed by an else statement. The else clause is executed only when the if statement’s condition is False. In plain English, an else statement could be read as, “If this condition is true, execute this code. Or else, execute that code.” An else statement doesn’t have a condition, and in code, an else statement always consists of the following:

• The else keyword
• A colon
• Starting on the next line, an indented block of code (called the else clause)

While only one of the if or else clauses will execute, you may have a case where you want one of many possible clauses to execute. The elif statement is an “else if” statement that always follows an if or another elif statement. It provides another condition that is checked only if any of the previous conditions were False. In code, an elif statement always consists of the following:

• The elif keyword
• A condition (that is, an expression that evaluates to True or False)
• A colon
• Starting on the next line, an indented block of code (called the elif clause)

One more topic you’ll need to understand before you can begin writing programs in earnest is the list data type and its cousin, the tuple. Lists and tuples can contain multiple values, which makes it easier to write programs that handle large amounts of data. And since lists themselves can contain other lists, you can use them to arrange data into hierarchical structures.

In this chapter, I’ll discuss the basics of lists. I’ll also teach you about methods, which are functions that are tied to values of a certain data type. Then I’ll briefly cover the list-like tuple and string data types and how they compare to list values.

A list is a value that contains multiple values in an ordered sequence. The term list value refers to the list itself (which is a value that can be stored in a variable or passed to a function like any other value), not the values inside the list value. A list value looks like this: ['cat', 'bat', 'rat', 'elephant']. Just as string values are typed with quote characters to mark where the string begins and ends, a list begins with an opening square bracket and ends with a closing square bracket, []. Values inside the list are also called items. Items are separated with commas (that is, they are comma-delimited). For example, enter the following into the interactive shell:

>>> [1, 2, 3]
[1, 2, 3]
>>> ['cat', 'bat', 'rat', 'elephant']
['cat', 'bat', 'rat', 'elephant']
>>> ['hello', 3.1415, True, None, 42]
['hello', 3.1415, True, None, 42]
>>> spam = ['cat', 'bat', 'rat', 'elephant']
>>> spam
['cat', 'bat', 'rat', 'elephant']

The spam variable (Line 7) is still assigned only one value: the list value. But the list value itself contains other values. The value [ ] is an empty list that contains no values, similar to ' ', the empty string.

Say you have the list ['cat', 'bat', 'rat', 'elephant'] stored in a variable named spam. The Python code spam[0] would evaluate to 'cat', and spam[1] would evaluate to 'bat', and so on. The integer inside the square brackets that follows the list is called an index. The first value in the list is at index 0, the second value is at index 1, the third value is at index 2, and so on. Figure 4-1 shows a list value assigned to spam, along with what the index expressions would evaluate to.

A list value stored in the variable spam, showing which value each index refers to Figure 4-1. A list value stored in the variable spam, showing which value each index refers to

For example, type the following expressions into the interactive shell. Start by assigning a list to the variable spam.

>>> spam = ['cat', 'bat', 'rat', 'elephant']
>>> spam[0]
'cat'
>>> spam[1]
'bat'
>>> spam[2]
'rat'
>>> spam[3]
'elephant'
>>> ['cat', 'bat', 'rat', 'elephant'][3]
'elephant'
 >>> 'Hello ' + spam[0]
'Hello cat'
 >>> 'The ' + spam[1] + ' ate the ' + spam[0] + '.'
 'The bat ate the cat.'

Notice that the expression 'Hello ' + spam[0] (Line 12) evaluates to 'Hello ' + 'cat' becausespam[0] evaluates to the string 'cat'. This expression, in turn, evaluates to the string value 'Hello cat' (Line 13).

Python will give you an IndexError error message if you use an index that exceeds the number of values in your list value.

>>> spam = ['cat', 'bat', 'rat', 'elephant']
>>> spam[10000]
Traceback (most recent call last):
File "<pyshell#9>", line 1, in <module>
spam[10000]

IndexError: list index out of range Indexes can be only integer values, not floats. The following example will cause a TypeError error:

>>> spam = ['cat', 'bat', 'rat', 'elephant']
>>> spam[1]
'bat'
>>> spam[1.0]
Traceback (most recent call last):
File "<pyshell#13>", line 1, in <module>
spam[1.0]
TypeError: list indices must be integers, not float
>>> spam[int(1.0)]
'bat'

Lists can also contain other list values. The values in these lists of lists can be accessed using multiple indexes, like so:

>>>spam = [['cat', 'bat'], [10, 20, 30, 40, 50]]
>>> spam[0]
['cat', 'bat']
>>> spam[0][1]
'bat'
>>> spam[1][4]
50

The first index dictates which list value to use, and the second indicates the value within the list value. For example, spam[0][1] prints 'bat', the second value in the first list. If you only use one index, the program will print the full list value at that index.

While indexes start at 0 and go up, you can also use negative integers for the index. The integer value-1 refers to the last index in a list, the value -2 refers to the second-to-last index in a list, and so on. Enter the following into the interactive shell:

>>> spam = ['cat', 'bat', 'rat', 'elephant']
>>> spam[-1]
'elephant'
>>> spam[-3]
'bat'
>>> 'The ' + spam[-1] + ' is afraid of the ' + spam[-3] + '.'
'The elephant is afraid of the bat.'

Just as an index can get a single value from a list, a slice can get several values from a list, in the form of a new list. A slice is typed between square brackets, like an index, but it has two integers separated by a colon. Notice the difference between indexes and slices.

spam[2] is a list with an index (one integer).

spam[1:4] is a list with a slice (two integers).

In a slice, the first integer is the index where the slice starts. The second integer is the index where the slice ends. A slice goes up to, but will not include, the value at the second index. A slice evaluates to a new list value. Enter the following into the interactive shell:

>>> spam = ['cat', 'bat', 'rat', 'elephant']
>>> spam[0:4]
['cat', 'bat', 'rat', 'elephant']
>>> spam[1:3]
['bat', 'rat']
>>> spam[0:-1]
['cat', 'bat', 'rat']

As a shortcut, you can leave out one or both of the indexes on either side of the colon in the slice. Leaving out the first index is the same as using 0, or the beginning of the list. Leaving out the second index is the same as using the length of the list, which will slice to the end of the list. Enter the following into the interactive shell:

>>> spam = ['cat', 'bat', 'rat', 'elephant']
>>> spam[:2]
['cat', 'bat']
>>> spam[1:]
['bat', 'rat', 'elephant']
>>> spam[:]
['cat', 'bat', 'rat', 'elephant']

The len() function will return the number of values that are in a list value passed to it, just like it can count the number of characters in a string value. Enter the following into the interactive shell:

>>> spam = ['cat', 'dog', 'moose']
>>> len(spam)
3

Normally a variable name goes on the left side of an assignment statement, like spam = 42. However, you can also use an index of a list to change the value at that index. For example, spam[1] = 'aardvark' means “Assign the value at index 1 in the list spam to the string 'aardvark'.” Enter the following into the interactive shell:

>>>spam = ['cat', 'bat', 'rat', 'elephant']
>>> spam[1] = 'aardvark'
>>> spam
['cat', 'aardvark', 'rat', 'elephant']
>>> spam[2] = spam[1]
>>> spam
['cat', 'aardvark', 'aardvark', 'elephant']
>>> spam[-1] = 12345
>>> spam
['cat', 'aardvark', 'aardvark', 12345]

The + operator can combine two lists to create a new list value in the same way it combines two strings into a new string value. The * operator can also be used with a list and an integer value to replicate the list. Enter the following into the interactive shell:

>>> [1, 2, 3] + ['A', 'B', 'C']
[1, 2, 3, 'A', 'B', 'C']
>>> ['X', 'Y', 'Z'] * 3
['X', 'Y', 'Z', 'X', 'Y', 'Z', 'X', 'Y', 'Z']
>>> spam = [1, 2, 3]
>>> spam = spam + ['A', 'B', 'C']
>>> spam
[1, 2, 3, 'A', 'B', 'C']

The del statement will delete values at an index in a list. All of the values in the list after the deleted value will be moved up one index. For example, enter the following into the interactive shell:

>>> spam = ['cat', 'bat', 'rat', 'elephant']
>>> del spam[2]
>>> spam
['cat', 'bat', 'elephant']
>>> del spam[2]
>>> spam
['cat', 'bat']

The del statement can also be used on a simple variable to delete it, as if it were an “unassignment” statement. If you try to use the variable after deleting it, you will get a NameError error because the variable no longer exists.

In practice, you almost never need to delete simple variables. The del statement is mostly used to delete values from lists.

A method is the same thing as a function, except it is “called on” a value. For example, if a list value were stored in spam, you would call the index() list method (which I’ll explain next) on that list like so: spam.index('hello'). The method part comes after the value, separated by a period.

Each data type has its own set of methods. The list data type, for example, has several useful methods for finding, adding, removing, and otherwise manipulating values in a list.

List values have an index() method that can be passed a value, and if that value exists in the list, the index of the value is returned. If the value isn’t in the list, then Python produces a ValueError error. Enter the following into the interactive shell:

>>> spam = ['hello', 'hi', 'howdy', 'heyas']
>>> spam.index('hello')
0
>>> spam.index('heyas')
3
>>> spam.index('howdy howdy howdy')
Traceback (most recent call last):
  File "<pyshell#31>", line 1, in <module>
    spam.index('howdy howdy howdy')
ValueError: 'howdy howdy howdy' is not in list

When there are duplicates of the value in the list, the index of its first appearance is returned. Enter the following into the interactive shell, and notice that index() returns 1, not 3:

>>> spam = ['Zophie', 'Pooka', 'Fat-tail', 'Pooka']
>>> spam.index('Pooka')
1

To add new values to a list, use the append() and insert() methods. Enter the following into the interactive shell to call the append() method on a list value stored in the variable spam:

>>> spam = ['cat', 'dog', 'bat']
>>> spam.append('moose')
>>> spam
['cat', 'dog', 'bat', 'moose']

The previous append() method call adds the argument to the end of the list. The insert() method can insert a value at any index in the list. The first argument to insert() is the index for the new value, and the second argument is the new value to be inserted. Enter the following into the interactive shell:

>>> spam = ['cat', 'dog', 'bat']
>>> spam.insert(1, 'chicken')
>>> spam
['cat', 'chicken', 'dog', 'bat']

Notice that the code is spam.append('moose') and spam.insert(1, 'chicken'), not spam = spam.append('moose') and spam = spam.insert(1, 'chicken'). Neither append() nor insert() gives the new value of spam as its return value. (In fact, the return value of append() and insert() is None, so you definitely wouldn’t want to store this as the new variable value.) Rather, the list is modified >in place. Modifying a list in place is covered in more detail later in Mutable and Immutable Data Types (Links to an external site.).

Methods belong to a single data type. The append() and insert() methods are list methods and can be called only on list values, not on other values such as strings or integers. Enter the following into the interactive shell, and note the AttributeError error messages that show up:

>>> eggs = 'hello'
>>> eggs.append('world')
Traceback (most recent call last):
  File "<pyshell#19>", line 1, in <module>
   eggs.append('world')
AttributeError: 'str' object has no attribute 'append'
>>> bacon = 42
>>> bacon.insert(1, 'world')
 Traceback (most recent call last):
 File "<pyshell#22>", line 1, in <module>
  bacon.insert(1, 'world')
AttributeError: 'int' object has no attribute 'insert'

The remove() method is passed the value to be removed from the list it is called on. Enter the following into the interactive shell:

>>> spam = ['cat', 'bat', 'rat', 'elephant']
>>>spam.remove('bat')
>>> spam
['cat', 'rat', 'elephant']

Attempting to delete a value that does not exist in the list will result in a ValueError error. For example, enter the following into the interactive shell and notice the error that is displayed:

>>> spam = ['cat', 'bat', 'rat', 'elephant']
>>> spam.remove('chicken')
Traceback (most recent call last):
 File "<pyshell#11>", line 1, in <module>
   spam.remove('chicken')
ValueError: list.remove(x): x not in list

If the value appears multiple times in the list, only the first instance of the value will be removed. Enter the following into the interactive shell:

>>> spam = ['cat', 'bat', 'rat', 'cat', 'hat', 'cat']
>>> spam.remove('cat')
>>> spam
['bat', 'rat', 'cat', 'hat', 'cat']

The del statement is good to use when you know the index of the value you want to remove from the list. The remove() method is good when you know the value you want to remove from the list.

Lists of number values or lists of strings can be sorted with the sort() method. For example, enter the following into the interactive shell:

>>> spam = [2, 5, 3.14, 1, -7]
>>> spam.sort()
>>> spam
[-7, 1, 2, 3.14, 5]
>>> spam = ['ants', 'cats', 'dogs', 'badgers', 'elephants']
>>> spam.sort()
>>> spam
['ants', 'badgers', 'cats', 'dogs', 'elephants']

You can also pass True for the reverse keyword argument to have sort() sort the values in reverse order. Enter the following into the interactive shell:

>>> spam.sort(reverse=True)
>>> spam
['elephants', 'dogs', 'cats', 'badgers', 'ants']

There are three things you should note about the sort() method. First, the sort() method sorts the list in place; don’t try to capture the return value by writing code like spam = spam.sort().

Second, you cannot sort lists that have both number values and string values in them, since Python doesn’t know how to compare these values. Type the following into the interactive shell and notice the TypeError error:

>>> spam = [1, 3, 2, 4, 'Alice', 'Bob']
>>> spam.sort()
Traceback (most recent call last):
  File "<pyshell#70>", line 1, in <module>
    spam.sort()
TypeError: unorderable types: str() < int()

Third, sort() uses “ASCIIbetical order” rather than actual alphabetical order for sorting strings. This means uppercase letters come before lowercase letters. Therefore, the lowercase a is sorted so that it comes after the uppercase Z. For an example, enter the following into the interactive shell:

>>> spam = ['Alice', 'ants', 'Bob', 'badgers', 'Carol', 'cats']
>>> spam.sort()
>>> spam
['Alice', 'Bob', 'Carol', 'ants', 'badgers', 'cats']

If you need to sort the values in regular alphabetical order, pass str. lower for the key keyword argument in the sort() method call.

>>> spam = ['a', 'z', 'A', 'Z']
>>> spam.sort(key=str.lower)
>>> spam
['a', 'A', 'z', 'Z']

This causes the sort() function to treat all the items in the list as if they were lowercase without actually changing the values in the list.

https://google.github.io/styleguide/pyguide.html#383-functions-and-methods

https://docs.python.org/3/library/math.html

https://docs.python.org/3/library/functions.html