58 Defining and Calling Python Functions

### **Defining and Calling Python Functions**

In Python, a **function** is a block of reusable code that performs a specific task. Functions help organize code, make it more readable, and allow for reusability. Defining and calling functions is a fundamental concept in Python programming.

### **1. Defining a Function**
To define a function in Python, we use the `def` keyword, followed by the function name and parentheses `()` that may contain parameters. The function body is indented under the `def` line.

**Syntax**:
```python
def function_name(parameters):
# Code block
return value # (Optional)
```

- **`function_name`**: The name of the function.
- **`parameters`**: Optional values passed into the function (also called arguments).
- **`return`**: Optionally returns a value from the function. If no return is specified, the function returns `None`.

### **Example: Defining a Simple Function**
```python
def greet():
print("Hello, welcome to Python!")
```

This defines a function named `greet` that, when called, prints a greeting message.

### **2. Calling a Function**
Once defined, you can **call** a function by using its name followed by parentheses. If the function has parameters, you pass arguments inside the parentheses.

**Syntax**:
```python
function_name(arguments) # Calling the function
```

### **Example: Calling a Function**
```python
greet() # Calls the greet function and prints the message
```

### **3. Functions with Parameters**
Functions can take **parameters** (input values) to make them more flexible and reusable.

**Syntax**:
```python
def greet(name):
print(f"Hello, {name}!")
```

You can call this function and pass an argument (value) for the parameter `name`.

**Example**:
```python
greet("Alice") # Prints: Hello, Alice!
greet("Bob") # Prints: Hello, Bob!
```

### **4. Functions with Return Values**
Functions can return a value using the `return` keyword. The returned value can then be used in other parts of your program.

**Syntax**:
```python
def add(x, y):
return x + y
```

**Example**:
```python
result = add(3, 5) # Calls the add function and stores the result
print(result) # Prints: 8
```

### **5. Example: A Complete Function with Parameters and Return**
```python
def multiply(a, b):
return a * b

result = multiply(4, 6)
print(result) # Prints: 24
```

### **Summary**:
- **Defining functions**: Use `def` followed by the function name and parameters.
- **Calling functions**: Use the function name followed by parentheses.
- **Parameters**: Functions can take input values (arguments) to make them more flexible.
- **Return values**: Functions can return a result using the `return` keyword, which can be used elsewhere in the code.

Functions are a core concept in Python, helping to organize code and improve its reusability and readability.