Python How to Convert String to Float Easily

Use the built-in float() function to convert a string to a float in Python like this: float('3.14').

Examples

Input'3.14'

Output3.14

Input'0.001'

Output0.001

Input'-123.456'

Output-123.456

How to Think About It

To convert a string to a float, think of the string as a number written in text form. You want to change it into a number your program can use for math. Python has a simple tool called float() that reads the string and turns it into a decimal number.

Algorithm

1

Get the string input that represents a number.

2

Use the <code>float()</code> function to convert the string to a floating-point number.

3

Return or use the converted float value.

Code

python

number_str = '3.14'
number_float = float(number_str)
print(number_float)

Output

3.14

Dry Run

Let's trace converting the string '3.14' to a float.

1

Input string

number_str = '3.14'

2

Convert to float

number_float = float('3.14') => 3.14

3

Print result

print(3.14) outputs 3.14

Step	Value
Input string	'3.14'
After conversion	3.14
Output	3.14

Why This Works

Step 1: Using float() function

The float() function reads the string and converts it to a decimal number type.

Step 2: String must be a valid number

The string should look like a number (digits, optional decimal point, optional sign) for float() to work.

Step 3: Result is a float type

After conversion, the value is a float number that can be used in math operations.

Alternative Approaches

Using eval() function

python

number_str = '3.14'
number_float = eval(number_str)
print(number_float)

Works but is unsafe if input is untrusted because eval() runs any code.

Using decimal.Decimal for precise decimal

python

from decimal import Decimal
number_str = '3.14'
number_decimal = Decimal(number_str)
print(number_decimal)

Gives more precise decimal numbers but result is not a float type.

Complexity: O(1) time, O(1) space

Time Complexity

Conversion with float() is a simple parsing operation that runs in constant time.

Space Complexity

No extra memory is needed besides the output float, so space is constant.

Which Approach is Fastest?

float() is the fastest and safest for converting strings to floats compared to alternatives like eval() or Decimal.

Approach	Time	Space	Best For
float()	O(1)	O(1)	Simple, safe string to float conversion
eval()	O(1)	O(1)	Unsafe, runs code, avoid for untrusted input
decimal.Decimal	O(1)	O(1)	Precise decimal numbers, not float type