Python round() is a built-in function that allows us to round a number in Python. It’s a straightforward function that comes in handy when dealing with decimal numbers or floating-point numbers.

In this article, we’ll take a closer look at the Python round() function and explore its various use cases.

## Syntax of Python round() Function

Before diving into the different scenarios in which the round() function is useful, let’s look at its syntax. The Python round() function receives a number as its input and rounds it to the nearest integer.

### The general syntax is:

round(number[,ndigits])

Here, the number argument is the input number you want to round, and the optional ndigits argument specifies the number of digits to round to. If you omit the ndigits argument, the Python round() function will round the number to the nearest integer.

## Rounding to Nearest Integer

One of the most common use cases of the Python round() function is rounding a decimal number to the nearest integer. Consider the following example:

```
n = 4.7
rounded_number = round(n)
print(rounded_number)
```

In this example, we have assigned the number 4.7 to the variable n and rounded it using the Python round() function. The output of the code is 5 since 4.7 is closer to 5 than it is to 4.

## Rounding to Even Side

The Python round() function has an interesting behavior when it comes to rounding numbers that end in .5. By default, the function rounds such numbers to the nearest even integer.

### For example:

```
n = 3.5
rounded_number = round(n)
print(rounded_number)
```

In this example, the output of the code is 4 since 3.5 is closer to 4 than it is to 3. But what if the number ends in .5 and the two closest integers are equidistant?

Here’s an example:

```
n = 4.5
rounded_number = round(n)
print(rounded_number)
```

In this case, the output of the code is 4 since, by default, Python rounds to the even side when the number ends in .5.

### Rounding with ndigit=None

When called without an ndigit argument, the Python round() function will round the input number to the nearest integer.

### Consider the following example:

```
n = 3.14159
rounded_number = round(n)
print(rounded_number)
```

In this example, the output of the code is 3 since the input number is closer to 3 than any other integer.

### Rounding with ndigit < 0

We can use the ndigit argument to specify the number of digits to round to.

If we set ndigit to a negative number, Python rounds the number of places to the left of the decimal point. Consider the following example:

```
n = 12345.6789
rounded_number = round(n, -2)
print(rounded_number)
```

In this example, we’ve rounded n to the nearest hundreds place. The output of the code is 12300 since 12345.6789 is closer to 12300 than it is to 12400.

## Anomalies on Rounding with Floating-Point Numbers

When working with floating-point numbers, we might encounter some unexpected results when rounding. This is due to how computers store floating-point numbers in binary.

### For example:

```
n = 1.1 + 2.2
rounded_number = round(n, 1)
print(rounded_number)
```

In this example, we’ve added 1.1 and 2.2, which should give us 3.3. However, because of how floating-point numbers are stored in binary, the result is actually slightly greater than 3.3. As a result, when we round to the nearest 0.1, we get 3.4 instead of 3.3.

## Examples of Python round()

Now that we’ve explored the different scenarios in which the Python round() function is useful, here are some examples of the function in action.

### Integer Rounding

```
n = 3.9
rounded_number = round(n)
print(rounded_number)
```

Output: 4

### Rounding to Even Side

```
n = 2.5
rounded_number = round(n)
print(rounded_number)
```

Output: 2

### Rounding with ndigit=None

```
n = 6.77
rounded_number = round(n)
print(rounded_number)
```

Output: 7

### Rounding with ndigit < 0

```
n = 1234.5678
rounded_number = round(n, -2)
print(rounded_number)
```

Output: 1200

## Conclusion

In conclusion, the Python round() function is a built-in function that comes in handy when working with decimal or floating-point numbers. It allows us to round numbers to the nearest integer or to a specified number of digits.

It’s important to keep in mind that when working with floating-point numbers, we might encounter some unexpected results due to how computers store them in binary. The Python round() function is a useful tool that can help us solve various rounding problems with ease.

## Using Python round() on Custom Objects

The Python round() function can not only be used on numerical values but can also be used on custom objects. However, to achieve this, we need to override the __round__() dunder method.

Dunder methods, also known as magic methods, are special methods in Python that have a double underscore prefix and suffix. These methods are used to mimic built-in Python behavior or to customize object behavior.

__round__() is one such dunder method that allows us to override the default rounding behavior of Python for custom objects. When we use the Python round() function on a custom object, Python will first look for the __round__() dunder method within our object.

If this method is defined, then Python will use it to round the object. If it’s not defined, Python will raise a TypeError.

### Overriding __round__() Dunder Method

Let’s take a closer look at how we can override the __round__() dunder method to use Python round() on custom objects. We start by defining a custom class and including the __round__() method within it.

The __round__() method should take a single argument, ndigits, which can be used to customize the rounding behavior. Here’s an example:

```
class Product:
def __init__(self, price, tax_rate):
self.price = price
self.tax_rate = tax_rate
def __round__(self, ndigits=0):
rounded_price = round(self.price * (1 + self.tax_rate), ndigits)
return rounded_price
product = Product(10, 0.12)
rounded_price = round(product, 2)
print(rounded_price)
```

In this example, we first create an instance of the Product class with a price of 10 and a tax rate of 0.12. We then use the Python round() function to round the product’s price to two decimal places.

The output of the code is 11.20 since the rounded price is 10 * (1 + 0.12) rounded to two decimal places. With this example, you can see how using the __round__() dunder method can customize the rounding behavior of custom objects and make them compatible with the Python round() function.

## Conclusion

In conclusion, the Python round() function is a powerful built-in function that makes rounding numbers and values easy in Python. It can round decimals to integers, round to the nearest even integer, round to a specific number of decimal places or to the left of the decimal point, and is useful in many different rounding scenarios.

When it comes to custom objects, we can use the __round__() dunder method to override the default rounding behavior of Python for our objects. With this method, we can customize the function, so it rounds our desired value based on our definition.

Python round() is a handy tool that comes in handy in multiple scenarios and can greatly simplify code when working with mathematical calculations. Thanks to the various functionalities it supports, we can create cleaner, more efficient code with fewer mathematical operations.

In summary, the Python round() function is an essential built-in function that simplifies rounding operations on numerical values. It comes with various functionalities, including rounding to nearest integers, rounding to even side, rounding with ndigits=None, and rounding with ndigits < 0.

However, when dealing with custom objects, Python can only use the __round__() dunder method to carry out the function’s rounding operation. The importance of Python round() lies in simplifying mathematical operations, and with some significant takeaways, including providing cleaner and more efficient code.

Overall, Python round() undeniably saves time, increases productivity, and in a nutshell, makes programming more enjoyable.