Working with variables
Remember that values are assigned to variables. Think of a variable as a named container for a value.
Storing values in variables
Use variables to store and refer to data in your programs.
The single equal = syntax assigns a value to a variable.
planet = 'Neptune' # string
orbital_period_years = 165 # integer
mass_ratio_to_earth = 17.15 # float
is_inner_planet = False # boolean
A variable can hold any type of value.
Checking the type of a variable
Python programmers often rely on variable names and context about the problem and solution domain to know what data type a variable’s value should be.
You can also use the built-in method type() to check the type of any value.
planet = 'Neptune'
type(planet) # <class 'str'>
orbital_period_years = 165
type(orbital_period_years) # <class 'int'>
mass_ratio_to_earth = 17.15
type(mass_ratio_to_earth) # <class 'float'>
is_inner_planet = False
type(is_inner_planet) # <class 'bool'>
Variables do not specify a data type. Variables do not enforce a data type.
A variable can later be reassigned to another value, even of a different type.
planet = 'Neptune'
type(planet) # <class 'str'>
planet = 8
type(planet) # <class 'int'>
# planet = 8
# planet was a string, then it was reassigned to an integer
Using the None object
If you plan to assign a value to a variable later in your program, try to choose a sensible default value.
# surely, there are downsides to defaulting someone's age to 21
your_age = 12
# later determine the user's actual age
You can also use a special object None to indicate that the variable has not been assigned yet
# wouldn't want to assume they are 12, either
your_age = None
# later determine the user's actual age
type(your_age) # <class 'NoneType'>
In fact, the None object is its own type.
Understanding types and classes
You may have noticed that the type() function shows that these values belong to their own class. For many programs you write in Python, it is okay to not worry about the difference between the terms type and class. If you are curious, you can read more about it in this StackOverflow article.
A class is a categorization of objects that all behave similarly. Based on their class, objects can take on certain values, have certain attributes, and work with certain methods.
Programmers can even define their own custom classes. For now, you can still accomplish a great deal using built-in classes.
Converting values to other types
In Python, there are functions that allow you to convert compatible values to another type. The class names returned by the type() function are the same names as the conversion functions for that type.
# convert a numerical string to an integer
int('24') # = 24
# convert a numerical float to a float
float('6.777') # = 6.777
# convert a float to an integer
# note: this conversion rounds towards zero
int(3.6) # = 3
int(3.2) # = 3
int(-7.6) # = -7
# convert multiple numbers into a bigger string
str(5) + '/' + str(2018) # = '5/2018'
The truthiness or falsiness of a value is a term that indicates what kind of boolean it can be converted into.
# zero is the only falsy numerical value
bool(0) # = False
bool(0.0) # = False
bool(2345) # = True
# even negatives are truthy
bool(-500) # = True
# empty strings are the only falsy strings
bool('') # = False
bool('ok') # = True