Parse decimal input
Convert request, environment, or command-line text into numeric values before calculations.
Python · Class
float
float constructs a floating-point number from a number or string. It also accepts no argument, returning 0.0.
How it works
float constructs a floating-point value from a number or a string. A string may contain a signed decimal value with surrounding whitespace; omitting the argument produces 0.0.
Syntax
float(x=0.0)Parameters
| Name | Type | Description |
|---|---|---|
x | number or string | A number or string to convert; omit it to obtain 0.0. |
Returns
A floating-point number; with no argument, 0.0.
Key points
- The built-in constructs a floating-point number from numeric or textual input.
- Calling float without an argument yields 0.0.
- A string may begin with either sign; plus does not change the magnitude.
- Scientific notation in a string is accepted and converted to its numeric value.
- The documented string forms include signed infinity, which produces negative infinity.
Common mistakes
- String input must represent a decimal number, with an optional sign and surrounding whitespace; invalid text cannot be converted.
- Integer and floating-point inputs preserve their value only within Python's floating-point precision.
Example: Construct floating-point values
value = float("1.25")
whole = float(3)
default = float()
print(value, whole, default)The snippet converts a decimal string and an integer, then shows the zero-argument default.
When to use it
Numeric coercion
Normalize integer and floating-point values to one numeric representation.
Default initialization
Create a zero-valued floating-point default when an optional numeric value is absent.
Interview questions
QWhat does float() do?
float() creates a floating-point number from a number or string. With no argument, it returns 0.0.
Follow-up: What does float(3) do? It returns a floating-point value with the same value, subject to floating-point precision.
QWhat string formats can float accept?
A string argument may contain a decimal number, an optional leading sign, and whitespace. Examples include values such as '+1.23' and ' -12345'.
Follow-up: What happens with nonnumeric text? Conversion fails because the string does not describe a valid decimal number.
QHow does float handle numeric inputs?
For an integer or another floating-point value, float returns a floating-point number with the same value within Python's floating-point precision. No argument returns 0.0.
Follow-up: Why can decimal arithmetic differ from expectations? Floating-point precision limits how values are represented and preserved.
Related
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