5 Best Ways to Convert Python String to Float with Precision

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Convert Python String to Float with Precision

πŸ’‘ Problem Formulation: You’re working with Python and receive numerical data in string format, such as "123.456789". When converting this string to a floating-point number, you want to maintain its precision as much as possible. The desired output is a float with no loss of the intricacies, like converting "123.456789" into a float with the value 123.456789 and not something rounded like 123.46.

Method 1: Standard float() Conversion

The default method for converting a string to a float in Python is via the built-in function float(). It is straightforward and generally preserves precision for most practical use cases. However, it might not handle extremely large or small numbers well.

Here’s an example:

num_str = "123.456789"
num_float = float(num_str)

Output: 123.456789

This code snippet converts a string containing a decimal number into a floating-point number using Python’s built-in float() function. The precision is maintained in this example, but the method may exhibit floating-point errors when dealing with very large or small numbers.

Method 2: Using Decimal Module

For better precision handling, Python’s decimal module can be used. It provides support for fast correctly-rounded decimal floating point arithmetic. Though it may not be as fast as other methods, it provides more precision and is particularly useful for financial applications.

Here’s an example:

from decimal import Decimal

num_str = "123.456789"
num_decimal = Decimal(num_str)

Output: Decimal('123.456789')

This snippet uses the Decimal class to accurately represent the number in the string. The output is a Decimal object with exact precision. This is ideal for use cases where precision is crucial.

Method 3: Rounding with Built-in round()

The round() function can be used in combination with float() to maintain precision to a specified number of decimal places. This method provides a way to control the level of precision required.

Here’s an example:

num_str = "123.456789"
num_float = round(float(num_str), 6)

Output: 123.456789

The round() function rounds the float to 6 decimal places, the precision of the original string. This method allows you to specify the number of decimal places but can also round the number, potentially losing some precision.

Method 4: Formatting with f-strings or format()

Using Python’s formatted string literals (f-strings) or the format() function can control the precision of floating-point numbers when converting them to strings. This method is good for representing float values with a specific number of decimal places in string form.

Here’s an example:

num_str = "123.456789"
num_float = f"{float(num_str):.6f}"

Output: '123.456789'

This code demonstrates the use of f-strings to maintain the number of decimal places when displaying the floating-point number as a string. The :.6f format specifier tells Python to format the float with six decimal places.

Bonus One-Liner Method 5: Using numpy.float64()

numpy is a popular library for numerical computation in Python. numpy.float64() provides double-precision floating-point numbers which might be useful when higher precision is needed.

Here’s an example:

import numpy as np

num_str = "123.456789"
num_float64 = np.float64(num_str)

Output: 123.456789

Converting a string to a numpy.float64 object preserves precision, utilizing NumPy’s capabilities for handling large arrays and matrix operations efficiently. It’s especially useful in scientific computations.


  • Method 1: Standard float() Conversion. Simple and straightforward but may lack precision for edge cases.
  • Method 2: Using Decimal Module. Offers precise decimal arithmetic at the cost of performance.
  • Method 3: Rounding with Built-in round(). Allows precision control up to a certain number of decimal places, can result in rounding errors.
  • Method 4: Formatting with f-strings or format(). Excellent for string representations of floats, less suitable when the float itself is needed.
  • Bonus One-Liner Method 5: Using numpy.float64(). Provides high precision with the tradeoff of requiring an external library.