Definition of Variables
- Variables are Containers
- Used for storing data values.
Creating Variables
-
No Declaration Command
- Python does not require a specific command to declare variables.
- A variable is created when you assign a value to it.
-
Examples:
-
x = 5 y = "John" print(x) print(y)
-
-
Dynamic Typing
- Variables do not need to have a fixed type and can change types after being set.
- Example:
x = 4 # x is of type int x = "Sally" # x is now of type str print(x)
Casting Variables
- Specifying Data Type
- Use casting to define a variable's data type explicitly.
- Example:
x = str(3) # x will be '3' y = int(3) # y will be 3 z = float(3) # z will be 3.0
Getting the Type of a Variable
- Using
type()Function- Check the data type of a variable.
- Example:
x = 5 y = "John" print(type(x)) # Output: <class 'int'> print(type(y)) # Output: <class 'str'>
Strings and Quotes
- Single or Double Quotes
- String variables can use either single (
') or double (") quotes interchangeably. - Example:
x = "John" y = 'John'
- String variables can use either single (
Case Sensitivity
- Variable Names are Case-Sensitive
- Variables with different cases are treated as distinct variables.
- Example:
a = 4 A = "Sally" # A will not overwrite a
Exercise
- Question: What is a correct way to declare a Python variable?
- Correct Answer:
x = 5
- Correct Answer:
Definition of Variable Names
- Short or Descriptive
- Variable names can be short (e.g.,
x,y) or descriptive (e.g.,age,carname,total_volume).
- Variable names can be short (e.g.,
Rules for Python Variable Names
-
Starting Character
- Must start with a letter or an underscore (
_).
- Must start with a letter or an underscore (
-
Cannot Start with a Number
- A variable name cannot begin with a digit.
-
Allowed Characters
- Can only contain alphanumeric characters (
A-Z,a-z,0-9) and underscores (_).
- Can only contain alphanumeric characters (
-
Case Sensitivity
- Variable names are case-sensitive. Examples:
age,Age, andAGEare treated as different variables.
- Variable names are case-sensitive. Examples:
-
Python Keywords
- Variable names cannot be any of Python's reserved keywords.
Examples of Legal and Illegal Variable Names
Legal Variable Names:
myvar = "John"
my_var = "John"
_my_var = "John"
myVar = "John"
MYVAR = "John"
myvar2 = "John"
Illegal Variable Names:
2myvar = "John" # Cannot start with a number
my-var = "John" # Cannot contain special characters like "-"
my var = "John" # Cannot contain spaces
Reminder
- Case Sensitivity:
- Variable names like
myvar,MyVar, andMYVARare distinct due to Python's case sensitivity.
- Variable names like
Many Values to Multiple Variables
-
Assigning Multiple Values in One Line
- Python allows assigning multiple values to multiple variables in one line.
- Example:
x, y, z = "Orange", "Banana", "Cherry" print(x) # Orange print(y) # Banana print(z) # Cherry
-
Matching Variables and Values
- The number of variables must match the number of values; otherwise, an error will occur.
One Value to Multiple Variables
- Assigning the Same Value to Multiple Variables
- A single value can be assigned to multiple variables in one line.
- Example:
x = y = z = "Orange" print(x) # Orange print(y) # Orange print(z) # Orange
Unpack a Collection
-
Extracting Values from Collections
- Python allows extracting values from a collection (like a list or tuple) into variables, called unpacking.
- Example:
fruits = ["apple", "banana", "cherry"] x, y, z = fruits print(x) # apple print(y) # banana print(z) # cherry
-
Learn More About Unpacking
- You can learn more about unpacking in the Unpack Tuples Chapter.
Exercise
Question:
What is the correct syntax to assign the value 'Hello World' to 3 variables in one statement?
Options:
x, y, z = 'Hello World'x = y = z = 'Hello World'✅x|y|z = 'Hello World'
Output Variables
-
Using
print()to Output Variables- The
print()function can be used to display the value of a variable. - Example:
x = "Python is awesome" print(x)
- The
-
Outputting Multiple Variables
- Multiple variables can be output using commas within the
print()function. - Example:
x = "Python" y = "is" z = "awesome" print(x, y, z) # Python is awesome
- Multiple variables can be output using commas within the
-
Using the
+Operator to Output Variables- The
+operator can be used to concatenate strings. - Example:
x = "Python " y = "is " z = "awesome" print(x + y + z) # Python is awesome - Note: If there are no spaces within the string variables, the result will appear as a single concatenated string (e.g.,
"Pythonisawesome").
- The
Working with Numbers
-
Mathematical Operation with
+- The
+operator adds numbers when used with numeric variables. - Example:
x = 5 y = 10 print(x + y) # 15
- The
-
Error When Combining Strings and Numbers
- Combining a string and a number with the
+operator will result in a TypeError. - Example:
x = 5 y = "John" print(x + y) # Error
- Combining a string and a number with the
-
Using Commas for Different Data Types
- Using commas in the
print()function allows you to combine different data types. - Example:
x = 5 y = "John" print(x, y) # 5 John
- Using commas in the
Exercise
Question:
Consider the following code:
print('Hello', 'World')
What will be the printed result?
Options:
- Hello, World
- Hello World ✅
- HelloWorld
Global Variables
Definition of Global Variables
- Global Variables are created outside of a function and can be used both inside and outside of functions.
Using Global Variables Inside Functions
- A global variable can be accessed directly within a function.
- Example:
x = "awesome" def myfunc(): print("Python is " + x) myfunc() # Output: Python is awesome
- Example:
Local Variables with the Same Name as Global Variables
- If a variable with the same name as a global variable is defined inside a function, it becomes a local variable and does not affect the global variable.
- Example:
x = "awesome" def myfunc(): x = "fantastic" # Local variable print("Python is " + x) myfunc() # Output: Python is fantastic print("Python is " + x) # Output: Python is awesome
- Example:
The global Keyword
- Definition: The
globalkeyword is used to define or modify a global variable inside a function.
Creating a Global Variable Inside a Function
- Using
globalwithin a function creates a global variable that can be accessed outside of the function.- Example:
def myfunc(): global x x = "fantastic" myfunc() print("Python is " + x) # Output: Python is fantastic
- Example:
Modifying a Global Variable Inside a Function
- To change the value of an existing global variable inside a function, use the
globalkeyword. - Example:
x = "awesome" def myfunc(): global x x = "fantastic" myfunc() print("Python is " + x) # Output: Python is fantastic
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