Short Introduction to Programming in Python

Overview

Teaching: 30 min
Exercises: 5 min

Questions

How do I program in Python?

How can I represent my data in Python?

Objectives

Describe the advantages of using programming vs. completing repetitive tasks by hand.

Define the following data types in Python: strings, integers, and floats.

Perform mathematical operations in Python using basic operators.

Define the following as it relates to Python: lists, tuples, and dictionaries.

Interpreter

Python is an interpreted language which can be used in two ways:

“Interactively”: when you use it as an “advanced calculator” executing one command at a time. To start Python in this mode, execute python on the command line:

$ python

Python 3.5.1 (default, Oct 23 2015, 18:05:06)
[GCC 4.8.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>>

Chevrons >>> indicate an interactive prompt in Python, meaning that it is waiting for your input.

2 + 2

print("Hello World")

Hello World

“Scripting” Mode: executing a series of “commands” saved in text file, usually with a .py extension after the name of your file:

$ python my_script.py

Hello World

Introduction to Python built-in data types

Strings, integers, and floats

One of the most basic things we can do in Python is assign values to variables:

text = "Data Carpentry"  # An example of a string
number = 42  # An example of an integer
pi_value = 3.1415  # An example of a float

Here we’ve assigned data to the variables text, number and pi_value, using the assignment operator =. To review the value of a variable, we can type the name of the variable into the interpreter and press Return:

text

"Data Carpentry"

Everything in Python has a type. To get the type of something, we can pass it to the built-in function type:

type(text)

<class 'str'>

type(number)

<class 'int'>

type(pi_value)

<class 'float'>

The variable text is of type str, short for “string”. Strings hold sequences of characters, which can be letters, numbers, punctuation or more exotic forms of text (even emoji!).

We can also see the value of something using another built-in function, print:

print(text)

Data Carpentry

print(number)

This may seem redundant, but in fact it’s the only way to display output in a script:

example.py

# A Python script file
# Comments in Python start with #
# The next line assigns the string "Data Carpentry" to the variable "text".
text = "Data Carpentry"

# The next line does nothing!
text

# The next line uses the print function to print out the value we assigned to "text"
print(text)

Running the script

$ python example.py

Data Carpentry

Notice that “Data Carpentry” is printed only once.

Tip: print and type are built-in functions in Python. Later in this lesson, we will introduce methods and user-defined functions. The Python documentation is excellent for reference on the differences between them.

Operators

We can perform mathematical calculations in Python using the basic operators +, -, /, *, %:

2 + 2  # Addition

6 * 7  # Multiplication

2 ** 16  # Power

13 % 5  # Modulo

We can also use comparison and logic operators: <, >, ==, !=, <=, >= and statements of identity such as and, or, not. The data type returned by this is called a boolean.

3 > 4

False

True and True

True

True or False

True

True and False

False

Sequences: Lists and Tuples

Lists

Lists are a common data structure to hold an ordered sequence of elements. Each element can be accessed by an index. Note that Python indexes start with 0 instead of 1:

numbers = [1, 2, 3]
numbers[0]

A for loop can be used to access the elements in a list or other Python data structure one at a time:

for num in numbers:
    print(num)

1
2
3

Indentation is very important in Python. Note that the second line in the example above is indented. Just like three chevrons >>> indicate an interactive prompt in Python, the three dots ... are Python’s prompt for multiple lines. This is Python’s way of marking a block of code. [Note: you do not type >>> or ....]

To add elements to the end of a list, we can use the append method. Methods are a way to interact with an object (a list, for example). We can invoke a method using the dot . followed by the method name and a list of arguments in parentheses. Let’s look at an example using append:

numbers.append(4)
print(numbers)

[1, 2, 3, 4]

To find out what methods are available for an object, we can use the built-in help command:

help(numbers)

Help on list object:

class list(object)
 |  list() -> new empty list
 |  list(iterable) -> new list initialized from iterable's items
 ...

Tuples

A tuple is similar to a list in that it’s an ordered sequence of elements. However, tuples can not be changed once created (they are “immutable”). Tuples are created by placing comma-separated values inside parentheses ().

# Tuples use parentheses
a_tuple = (1, 2, 3)
another_tuple = ('blue', 'green', 'red')

# Note: lists use square brackets
a_list = [1, 2, 3]

Tuples vs. Lists

What happens when you execute a_list[1] = 5?

What happens when you execute a_tuple[2] = 5?

What does type(a_tuple) tell you about a_tuple?

Dictionaries

A dictionary is a container that holds pairs of objects - keys and values.

translation = {'one': 'first', 'two': 'second'}
translation['one']

'first'

Dictionaries work a lot like lists - except that you index them with keys. You can think about a key as a name or unique identifier for the value it corresponds to.

rev = {'first': 'one', 'second': 'two'}
rev['first']

'one'

To add an item to the dictionary we assign a value to a new key:

rev = {'first': 'one', 'second': 'two'}
rev['third'] = 'three'
rev

{'first': 'one', 'second': 'two', 'third': 'three'}

Using for loops with dictionaries is a little more complicated. We can do this in two ways:

for key, value in rev.items():
    print(key, '->', value)

'first' -> one
'second' -> two
'third' -> three

for key in rev.keys():
    print(key, '->', rev[key])

'first' -> one
'second' -> two
'third' -> three

Changing dictionaries

First, print the value of the rev dictionary to the screen.

Reassign the value that corresponds to the key second so that it no longer reads “two” but instead 2.

Print the value of rev to the screen again to see if the value has changed.

Functions

Defining a section of code as a function in Python is done using the def keyword. For example a function that takes two arguments and returns their sum can be defined as:

def add_function(a, b):
    result = a + b
    return result

z = add_function(20, 22)
print(z)

Key Points

Python is an interpreted language which can be used interactively (executing one command at a time) or in scripting mode (executing a series of commands saved in file).

One can assign a value to a variable in Python. Those variables can be of several types, such as string, integer, floating point and complex numbers.

Lists and tuples are similar in that they are ordered lists of elements; they differ in that a tuple is immutable (cannot be changed).

Dictionaries are data structures that provide mappings between keys and values.

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