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# The first letter of a variable identifier must be a letter or an underscore
my_number = 7
# Error: Variable names cannot start with a number

# Function identifier
def print_my_number(number):
    print("Value of my_number:", number)

# Print the value of the variable
print_my_number(my_number)

# What are Identifiers?

`Identifiers` are names used to label variables, functions, classes, modules, and other objects in a program.

Note: An object refers to anything that includes data and the behavior (methods) related to this data. In Python, everything is an object.

<br />

## Python Identifier Rules

1. `Starting Character Rule`: An identifier must start with a letter (A-Z, a-z) or an underscore (\_). It cannot start with a digit.

2. `Use of Characters, Digits, Underscore`: After the first character, it can be followed by letters, digits (0-9), or underscores (\_).

3. `Reserved Keywords Cannot Be Used`: Words like def and if, which are reserved for specific language functions, cannot be used as identifiers.

4. `Case Sensitivity`: Python identifiers are case-sensitive. `myname` and `myName` are considered different identifiers.

```python title="Identifier Examples"
# Valid identifier examples
my_variable = 10    # Variable name starts with a letter or underscore

def my_function():  # Function name starts with a letter or underscore
    print("Hello")

class MyClass:      # Class name starts with a letter or underscore
    pass


# Invalid identifier examples
2my_variable = 10   # Variable name cannot start with a digit

def if():           # 'if' keyword cannot be used as a function name
```

<br />

## Importance of Identifiers

Identifiers like variable, function, and class names should be descriptive and reflect their purpose in the code.

```python title="Function Example to Calculate Circle Area"
radius = 25  # Variable name representing the radius

def calculate_circle_area(radius): # Function name to calculate circle area
    return 3.14 * radius * radius
```

<br />

## Coding Practice

Try entering the highlighted `1_my_number = 7` on the practice screen.

You'll see an error occurs when a variable name starts with a digit.