Python Variables Definition Creation Casting Getting the Type of a Variable Case Sensitivity Output Local Global Variable Inside a Function Numbers

Definition of Variables

1. Variables are Containers
   • Used for storing data values.

---

Creating Variables

1. No Declaration Command

   • Python does not require a specific command to declare variables.
   • A variable is created when you assign a value to it.

2. Examples:

   • x = 5
     y = "John"
     print(x)
     print(y)
     

3. 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

1. 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

1. 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

1. Single or Double Quotes
   • String variables can use either single (') or double (") quotes interchangeably.
   • Example:

     x = "John"
     y = 'John'
     

---

Case Sensitivity

1. 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

---

Definition of Variable Names

1. Short or Descriptive
   • Variable names can be short (e.g., x, y) or descriptive (e.g., age, carname, total_volume).

---

Rules for Python Variable Names

1. Starting Character

   • Must start with a letter or an underscore (_).

2. Cannot Start with a Number

   • A variable name cannot begin with a digit.

3. Allowed Characters

   • Can only contain alphanumeric characters (A-Z, a-z, 0-9) and underscores (_).

4. Case Sensitivity

   • Variable names are case-sensitive. Examples:
     • age, Age, and AGE are treated as different variables.

5. 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, and MYVAR are distinct due to Python's case sensitivity.

---

---

Many Values to Multiple Variables

1. 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
     

2. Matching Variables and Values

   • The number of variables must match the number of values; otherwise, an error will occur.

---

One Value to Multiple Variables

1. 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

1. 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
     

2. 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:

1. x, y, z = 'Hello World'
2. x = y = z = 'Hello World' ✅
3. x|y|z = 'Hello World'

---

Output Variables

1. 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)
     

2. 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
     

3. 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").

---

Working with Numbers

1. Mathematical Operation with +

   • The + operator adds numbers when used with numeric variables.
   • Example:

     x = 5
     y = 10
     print(x + y)  # 15
     

2. 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
     

3. 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
     

---

Exercise

Question:
Consider the following code:

print('Hello', 'World')

What will be the printed result?

Options:

1. Hello, World
2. Hello World ✅
3. HelloWorld

Global Variables

---

Definition of Global Variables

1. Global Variables are created outside of a function and can be used both inside and outside of functions.

---

Using Global Variables Inside Functions

2. A global variable can be accessed directly within a function.
   • Example:

     x = "awesome"
     
     def myfunc():
         print("Python is " + x)
     
     myfunc()  # Output: Python is awesome
     

---

Local Variables with the Same Name as Global Variables

3. 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
     

---

The global Keyword

4. Definition: The global keyword is used to define or modify a global variable inside a function.

---

Creating a Global Variable Inside a Function

5. Using global within 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
     

---

Modifying a Global Variable Inside a Function

6. To change the value of an existing global variable inside a function, use the global keyword.
• Example:

  x = "awesome"
  
  def myfunc():
      global x
      x = "fantastic"
  
  myfunc()
  print("Python is " + x)  # Output: Python is fantastic
  

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