# Functions

## Defining Functions

* For defining a function in python we use def(define) followed by function\_name() and colon
* Make sure that the function name is meaningful and descriptive
* Function names are generally given in snake\_case in python

```python
def greet():
    print("Hi there")
    print("Welcome abroad")
    
greet()
```

## Arguments and Parameters

* A parameter is an input that we define for our function 
* In the below example, first\_name and last\_name are parameters 
* While an argument is an actual value for a given parameter ("Ankit", "Choudhary" are arguments)

```python
def greet(first_name,last_name):
    print(first_name + " " + last_name))
    print("Welcome aboard")  
    
greet("OoBA","Labs")
```

## Types of Functions

### Perform a Task

* Below is a sample function in python
* In Python, all functions by default return None values

```python
def greet(first_name,last_name):
    print("Hi {} {}".format(first_name,last_name)) #Python2
    #print(f"Hi {first_name} {last_name}") #Python3
    print("Welcome aboard")
       
greet("OoBA","Labs")
```

### Return a Value

```python
def greet(first_name,last_name):
    return first_name + " " + last_name

full_name = greet("Ankit","Choudhary")
print(full_name) # prints "Ankit Choudhary"
```

## Arguments in Function

* Specifying the name of a parameter with an argument is called a keyword argument
* For example: "number=1" is a keyword argument in the next example

### Keyword Argument

```python
def increment(number,by):
    return number + by

print(increment(number=1,by=2)) # prints 3
```

### Default Argument

* All the parameters which we define for a function are required by default
* We can make a parameter optional by specifying the default value while defining the function 
* In the below given example "by" is having 1 as default value if no value is passed as an argument
* All the optional parameters must be defined after the required parameters.

```python
def increment(number,by = 1):
    return number + by

print(increment(number=1)) # print 2
```

### Xargs, Variable number of arguments

* We can pass a variable number of arguments to a function by using \* before the parameter name
* The variable arguments will be stored in a tuple and can be iterated.
* **Note that** variable arguments parameter should be at the end of function parameters

```python
def multiply(x,*numbers):
    print(x) # prints 2
    print(numbers) # prints (3,4,5)
    total = 1
    for number in numbers:
        total *= number
    return total
    
print(multiply(2,3,4,5)) # prints 60
```

### XXargs

* We can also pass multiple "key: value" pairs by using \*\* before the parameter name
* **Note that** variable arguments parameter should be at the end of function parameters

```python
def save_user(number,**user):
    print(number) # prints 1
    print(user) # prints dictionary with all key value pairs
    print(user["name"]) # "John"
        
save_user(1,id =1, name="John", age=22)
```

## Scope of a Function

```python
def greet(name):
    message = "a"
        
def send_mail(name):
    message = "b"
```

### Global Variable

* A Global variable is will be accessible throughout the file and inside each and every function
* Global variables stay for a longer period of time in a program until it is garbage collected,  so we should not use them often
* If the same variable is defined in a function's scope, the local variable will override the global variable
* Here in function greet, the global variable "message" will be used While in the function send\_mail as message variable is defined locally so that will be used.

```python
messge = "global variable"
def greet(name):
    print(messge)
        
def send_mail(name):
    message = "local variable"
    print(message)
    
greet("OoBA")
send_mail("OoBA")
```


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