5 Best Ways to Convert a Python String to Float With 2 Decimals

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💡 Problem Formulation: You’ve been given a string representation of a number, say “123.4567“, and you need to convert this to a floating-point number rounded to two decimal places. Essentially, you want to transform the string into a value akin to 123.46. In this article, we’ll explore five effective methods to achieve this in Python.

Method 1: Using the round() Function After Conversion

The round() function is a built-in Python method that rounds a floating point number to a specified number of decimal places. In this method, we first convert the string to a float and then round it to two decimal places.

Here’s an example:

num_string = "123.4567"
num_float = float(num_string)
rounded_num = round(num_float, 2)

Output: 123.46

In the code above, we convert the string num_string to a float and store it in num_float. We then use round(num_float, 2) to round it to two decimal places and store the result in rounded_num.

Method 2: Formatting With ‘%’

The string formatting operation with ‘%’ allows you to format a number as a string and round it to a specified number of decimal places. After formatting, we convert the string back into a float.

Here’s an example:

num_string = "123.4567"
formatted_num = "%.2f" % float(num_string)
final_num = float(formatted_num)

Output: 123.46

This snippet first converts the string to a float, formats it to have two decimal places using the string format operator ‘%’, and then converts the formatted string back to a float, resulting in final_num.

Method 3: Using the format() Function

The format() function in Python is used to format a value into a specific format. In this case, we can format a number as a string with two decimal places, and then convert it back to a float.

Here’s an example:

num_string = "123.4567"
formatted_num = "{:.2f}".format(float(num_string))
final_num = float(formatted_num)

Output: 123.46

Here, we use "{:.2f}".format(float(num_string)) to create a string with two decimal places. We then convert that string into a float, which gives us the final output final_num.

Method 4: Using Decimal Module

The Decimal module provides support for fast correctly-rounded decimal floating point arithmetic. We can use this module to round a string to two decimal places accurately.

Here’s an example:

from decimal import Decimal, ROUND_HALF_UP

num_string = "123.4567"
decimal_num = Decimal(num_string).quantize(Decimal('0.00'), rounding=ROUND_HALF_UP)

Output: Decimal('123.46')

The code above converts the string to a Decimal object, and then the quantize method is used to round the number to two decimal places. Note that the output is a Decimal object; you can convert it to a float if needed.

Bonus One-Liner Method 5: Using List Comprehension and Join

This one-liner method combines list comprehension and the join method to create a formatted string that represents the number with two decimal places before converting it back to a float.

Here’s an example:

num_string = "123.4567"
final_num = float('.'.join([num_string.split('.')[0], num_string.split('.')[1][:2]]))

Output: 123.45

The code splits the number on the dot, truncates the decimal part to two characters using list slicing, and then joins it back into a string. The string is then converted into a float. This method does not perform rounding but truncation.

Summary/Discussion

  • Method 1: Using round(). Easy and straightforward. Reliable rounding.
  • Method 2: Formatting with ‘%’. Less common in modern Python code but still effective. Requires conversion back to float.
  • Method 3: Using format(). Pythonic and readable. Consistent with other string formatting needs.
  • Method 4: Using Decimal Module. Best for financial calculations where accuracy is crucial. Slightly more complex.
  • Method 5: List Comprehension and Join. Quick one-liner suitable for simple truncation. Does not round but rather truncates the number.