05_variables.md

Variables

In Python, you use variables to store data in your computer's memory. By storing data in memory, your application can access that data while running your application. If you have a number, for instance, in your Python code, but you don't save it to a variable, the Python interpreter will not remember that number as it continues reading the remainder of your. If, however, you save that number to a variable (e.g., x = 10), Python stores x in memory and assigns it a value of 10. With the value stored in memory, you can use it later in your code.

# example of storing a value in a variable and using it later
y = 20
print(10 + y)

>>> 30

You can store almost anything as a variable. You can store numbers, strings, dictionaries, lists, tuples, functions, and more. When storing data in memory, the syntax requires the variable on the left, followed by an equals sign, followed by the value to the right of the equals sign:

# example of variable where g is the variable and the string is the value
g = "Programming in Python is fun."

Variables are, well, variable. In Python, you can change the value of variables. For instance, if you initially declare a variable named x and set it equal to 10 (x = 10), you later can change the value of x to something else, like x = 21.

Because Python allows for dynamic typing, in addition to changing the the value, you can also change the type. Accordingly, instead of changing the value of x to another integer like 21, you can change it to some other data type, like a string. For instance, you can change it to x= "Ten".

Variables aren't always meant to be varied. Variables are much more complex than most beginners realize. Here is a short list of advice that can help you write better Python code and also to become better at reading Python code.

Variables:

• must be assigned a value before you can use the variable
• must start with _ or lowercase letter
• are case sensitive
• can include letters, _, or numbers
• can have their value changed
• can be set to any data type
• can be assigned to another variable
• can be set along with more than one other variable in a single expression
• should generally be snake_case
• should be IN_ALL_UPPERCASE to represent a constant (to indicate to other developers that the variable should not be changed, only read)
• should be in UpperCamelCase to represent a class
• should start with a double underscore to represent private variables (indicating to other developers to read but not set its value)

Examples of variables

You must assign a variable a value before you can use the variable

# example of assigning a value to a variable before using the variable
j = 10  
print(j)
>>> 10

You can change the value of a variable (hence the meaning of “variable”)

# example of changing a variable's value
j = 10 
print(j)
>>> 10

j = 20    
print(j)  
>>> 20

You can set variables equal to any data type

# example of setting a variable to a string
g = "this is a letter"   
print(g) 
>>> "this is a letter" 

You can assign a variable to a variable

# example of assigning a variable to a variable
g = 18
j = g    
print(j)
>>> 18

You can set multiple variables in one expression

# example of setting multiple variables in one expression
a, b, c = 1, 2, 3    
print(a)  
print(b)  
print(c)  
>>> 1
>>> 2
>>> 3

If a variable is assigned a value, you can use it

# example of usnig a variable after assigning it a value
k = 12  
print(k)
>>> 12
print(k + 1)  
>>> 13

If you have not assigned a value to a variable, you can't use it

# example of trying to use a variable before assigning it a value
k = k + 1  
print(k)  

>>> Traceback (most recent call last):
>>>   File "variables_data_types.py", line 62, in <module>
>>>     k = k + 1
>>> NameError: name 'k' is not defined

Casting

You can set a variable's type using one of the built-in Python constructor functions. The Python data types each have their own constructor function. The constructor function you use will convert the value into the corresponding data type. For instance, using Python's built-in float constructor function float() will turn into a float whatever value you pass into it (assuming it's a valid value for that function).

# example of using the float() constructor function
a = float(17)     # x's value is 17.0
b = float(17.3)   # y's value is 17.3
c = float("17")   # z will be 17.0

Here is a list of Python constructor functions:

# example of constructor function for integer
int(100)

# example of constructor function for float
float(100.1)

# example of constructor function for string
str("Python")

# example of constructor function for bool
bool(100) # True 
bool(0) # False

# example of constructor function for list
list(("Python"))

# example of constructor function for tuple
tuple(("Python"))

# example of constructor function for dict
dict(name="Python", success=True)

# example of constructor function for set
set(("Python"))

# example of constructor function for bytes
bytes(100)

None

At times you may want a variable to exist but to have an empty value. In those situations, you can assign a variable to be equal to None. That variable will represent an empty value. For those of you with JavaScript experience, None is equivalent to null.

# example of printing None
print(None, type(None))  
>>> None, <class 'NoneType'>