Short Introduction to Programming in Python
Last updated on 2023-06-13 | Edit this page
Estimated time: 35 minutes
Overview
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:
OUTPUT
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.
OUTPUT
4
OUTPUT
Hello World
- “Scripting” Mode: executing a series of “commands” saved in text
file, usually with a
.py
extension after the name of your file:
OUTPUT
Hello World
Introduction to variables in Python
Assigning values to variables
One of the most basic things we can do in Python is assign values to variables:
PYTHON
text = "Data Carpentry" # An example of assigning a value to a new text variable,
# also known as a string data type in Python
number = 42 # An example of assigning a numeric value, or an integer data type
pi_value = 3.1415 # An example of assigning a floating point value (the float data type)
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:
OUTPUT
"Data Carpentry"
Everything in Python has a type. To get the type of something, we can
pass it to the built-in function type
:
OUTPUT
<class 'str'>
OUTPUT
<class 'int'>
OUTPUT
<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
:
OUTPUT
Data Carpentry
OUTPUT
42
This may seem redundant, but in fact it’s the only way to display output in a script:
example.py
PYTHON
# 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
OUTPUT
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.
Tip: When editing scripts like example.py, be careful not to use word processors such as MS Word, as they may introduce extra information that confuses Python. In this lesson we will be using either Jupyter notebooks or the Spyder IDE, and for your everday work you may also choose any text editor such as Notepad++, VSCode, Vim, or Emacs.
Operators
We can perform mathematical calculations in Python using the basic
operators +, -, /, *, %
:
OUTPUT
4
OUTPUT
42
OUTPUT
65536
OUTPUT
3
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.
OUTPUT
False
OUTPUT
True
OUTPUT
True
OUTPUT
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:
OUTPUT
1
A for
loop can be used to access the elements in a list
or other Python data structure one at a time:
OUTPUT
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
:
OUTPUT
[1, 2, 3, 4]
To find out what methods are available for an object, we can use the
built-in help
command:
OUTPUT
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 ()
.
PYTHON
# 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 abouta_tuple
? - What information does the built-in function
len()
provide? Does it provide the same information on both tuples and lists? Does thehelp()
function confirm this?
- What happens when you execute
a_list[1] = 5
?
The second value in a_list
is replaced with
5
.
- What happens when you execute
a_tuple[2] = 5
?
ERROR
TypeError: 'tuple' object does not support item assignment
As a tuple is immutable, it does not support item assignment. Elements in a list can be altered individually.
- What does
type(a_tuple)
tell you abouta_tuple
?
OUTPUT
<class 'tuple'>
The function tells you that the variable a_tuple
is an
object of the class tuple
.
- What information does the built-in function
len()
provide? Does it provide the same information on both tuples and lists? Does thehelp()
function confirm this?
OUTPUT
3
OUTPUT
3
len()
tells us the length of an object. It works the
same for both lists and tuples, providing us with the number of entries
in each case.
OUTPUT
Help on built-in function len in module builtins:
len(obj, /)
Return the number of items in a container.
Lists and tuples are both types of container i.e. objects that can contain multiple items, the key difference being that lists are mutable i.e. they can be modified after they have been created, while tuples are not: their value cannot be modified, only overwritten.
Dictionaries
A dictionary is a container that holds pairs of objects - keys and values.
OUTPUT
'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.
OUTPUT
'one'
To add an item to the dictionary we assign a value to a new key:
OUTPUT
{'first': 'one', 'second': 'two', 'third': 'three'}
Using for
loops with dictionaries is a little more
complicated. We can do this in two ways:
OUTPUT
'first' -> one
'second' -> two
'third' -> three
or
OUTPUT
'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 instead2
. - 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:
OUTPUT
42
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.