Python Getting Started
Python Install
Many PCs and Macs will have python already installed.
To check if you have python installed on a Windows PC, search in the start bar for Python or run the following on the Command Line (cmd.exe):
C:\Users\Your Name>python --version
To check if you have python installed on a Linux or Mac, then on linux open the command line or on Mac open the Terminal and type:
python --version
If you find that you do not have python installed on your computer, then you can download it for free from the following website: https://www.python.org/
Python Quickstart
Python is an interpreted programming language, this means that as a developer you write Python (.py) files in a text editor and then put those files into the python interpreter to be executed.
The way to run a python file is like this on the command line:
C:\Users\Your Name>python helloworld.py
Where "helloworld.py" is the name of your python file.
Let's write our first Python file, called helloworld.py, which can be done in any text editor.
print("Hello, World!")
Python Variables
Creating Variables
Variables are containers for storing data values.
Unlike other programming languages, Python has no command for declaring a variable.
A variable is created the moment you first assign a value to it.
x = 5
y = "John"
print(x)
print(y)
Variable Names
A variable can have a short name (like x and y) or a more descriptive name (age, carname, total_volume). Rules for Python variables:
- A variable name must start with a letter or the underscore character
- A variable name cannot start with a number
- A variable name can only contain alpha-numeric characters and underscores (A-z, 0-9, and _ )
- Variable names are case-sensitive (age, Age and AGE are three different variables)
#Legal variable names:
myvar = "John"
my_var = "John"
_my_var = "John"
myVar = "John"
MYVAR = "John"
myvar2 = "John"
#Illegal variable names:
2myvar = "John"
my-var = "John"
my var = "John"
Output Variables
The Python print statement is often used to output variables.
To combine both text and a variable, Python uses the + character:
x = "awesome"
print("Python is " + x)
x = "Python is "
y = "awesome"
z = x + y
print(z)
Python List
Python Collections (Arrays)
There are four collection data types in the Python programming language:
- List is a collection which is ordered and changeable. Allows duplicate members.
- Tuple is a collection which is ordered and unchangeable. Allows duplicate members.
- Set is a collection which is unordered and unindexed. No duplicate members.
- Dictionary is a collection which is unordered, changeable and indexed. No duplicate members.
When choosing a collection type, it is useful to understand the properties of that type. Choosing the right type for a particular data set could mean retention of meaning, and, it could mean an increase in efficiency or security.
List
A list is a collection which is ordered and changeable. In Python lists are written with square brackets.
thislist = ["apple", "banana", "cherry"]
print(thislist)
Access Items
You access the list items by referring to the index number:
thislist = ["apple", "banana", "cherry"]
print(thislist[1])
Range of Indexes
You can specify a range of indexes by specifying where to start and where to end the range.
When specifying a range, the return value will be a new list with the specified items.
thislist = ["apple", "banana", "cherry", "orange", "kiwi", "melon", "mango"]
print(thislist[2:5])
Remember that the first item has index 0.
Change Item Value
To change the value of a specific item, refer to the index number:
thislist = ["apple", "banana", "cherry"]
thislist[1] = "blackcurrant"
print(thislist)
List Methods
Python has a set of built-in methods that you can use on lists.
| Method | Description |
|---|---|
| append() | Adds an element at the end of the list |
| clear() | Removes all the elements from the list |
| copy() | Returns a copy of the list |
| count() | Returns the number of elements with the specified value |
| extend() | Add the elements of a list (or any iterable), to the end of the current list |
| index() | Returns the index of the first element with the specified value |
| insert() | Adds an element at the specified position |
| pop() | Removes the element at the specified position |
| remove() | Removes the item with the specified value |
| sort() | Sorts the list |
Python Tuples
Tuple
A tuple is a collection which is ordered and unchangeable. In Python tuples are written with round brackets.
thistuple = ("apple", "banana", "cherry")
print(thistuple)
Change Tuple Values
Once a tuple is created, you cannot change its values. Tuples are unchangeable, or immutable as it also is called.
But there is a workaround. You can convert the tuple into a list, change the list, and convert the list back into a tuple.
x = ("apple", "banana", "cherry")
y = list(x)
y[1] = "kiwi"
x = tuple(y)
print(x)
Python Dictionaries
Dictionary
A dictionary is a collection which is unordered, changeable and indexed. In Python dictionaries are written with curly brackets, and they have keys and values.
thisdict = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}
print(thisdict)
Accessing Items
You can access the items of a dictionary by referring to its key name, inside square brackets:
x = thisdict["model"]
Change Values
You can change the value of a specific item by referring to its key name:
thisdict = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}
thisdict["year"] = 2018
Python While Loops
The While Loop
With the while loop we can execute a set of statements as long as a condition is true.
i = 1
while i < 6:
print(i)
i += 1
Remember to increment i, or else the loop will continue forever.
The else Statement
With the else statement we can run a block of code once when the condition no longer is true:
i = 1
while i < 6:
print(i)
i += 1
else:
print("i is no longer less than 6")
Python For Loops
For Loops
A for loop is used for iterating over a sequence (that is either a list, a tuple, a dictionary, a set, or a string).
This is less like the for keyword in other programming languages, and works more like an iterator method as found in other object-orientated programming languages.
With the for loop we can execute a set of statements, once for each item in a list, tuple, set etc.
fruits = ["apple", "banana", "cherry"]
for x in fruits:
print(x)
Else in For Loop
The else keyword in a for loop specifies a block of code to be executed when the loop is finished:
for x in range(6):
print(x)
else:
print("Finally finished!")
Python Functions
Creating a Function
A function is a block of code which only runs when it is called.
You can pass data, known as parameters, into a function.
A function can return data as a result.
In Python a function is defined using the def keyword:
def my_function():
print("Hello from a function")
Calling a Function
To call a function, use the function name followed by parenthesis:
def my_function():
print("Hello from a function")
my_function()
Reference
Disclaimer
The only purpose of this guide is to provide content to build up a technical documentation
page for the freeCodeCamp Curriculum.
To get more informations about Python or other programming languages check w3shcool.com