float() — Convert to Floating-Point

float() converts a possible value into a floating-point number (float).

This lesson focuses on converting integers, booleans, and numeric strings into decimal numbers, while understanding why complex values and non-numeric text fail.

What you'll learn

• The purpose of float()
• Convert int, bool, and numeric strings
• Understand why complex values fail
• Learn how string indexing can still produce valid floats

What does float() do?

float() converts one possible type of value into a floating-point number.

number = float(value)

The resulting value always has the float type, even when the number is whole.

Example 1 — Integer → Float (Possible)

An integer becomes the same number with .0.

a = 10
print(a, type(a))

b = float(a)
print(b, type(b))

print(float(100))

10 <class 'int'>
10.0 <class 'float'>
100.0

The value keeps the same magnitude — only its type changes.

Example 2 — Boolean → Float (Possible)

Booleans convert exactly like they did with int(), but the result is a float.

Boolean	float() result
True	1.0
False	0.0

print(float(True))    # 1.0
print(float(False))   # 0.0

Example 3 — Complex → Float (Not Possible)

Complex numbers are not real numbers for float().

a = 2 + 3j

# float(a)

TypeError: float() argument must be a string or a real number, not 'complex'

This is a TypeError because the type itself is unsupported.

Example 4 — String → Float

Unlike int(), float() accepts both integer strings and decimal strings.

Case 1 — Integer string (Possible)

print(float("12"))
print(float("123"))

12.0
123.0

Case 2 — Decimal string (Possible)

print(float("12.34"))
print(float("34.56"))

12.34
34.56

This is the biggest difference from int():

	Call	Result
Valid	float("12.34")	12.34
Invalid	int("12.34")	ValueError

Example 5 — Invalid string values

These are strings, but they are not numeric literals.

Input	Result
"True"	ValueError
"2+3j"	ValueError
"Python.Java"	ValueError

# float("True")
# float("2+3j")
# float("Python.Java")

Interactive playground

Select a source type and see whether float() succeeds.

Special case — Indexing a complex string

This often confuses beginners.

The entire string "2+3j" is not a float. But individual characters can be.

Index	Character	float(...)
0	"2"	2.0
1	"+"	ValueError
2	"3"	3.0
3	"j"	ValueError

a = "2+3j"

print(float(a[0]))   # 2.0
print(float(a[2]))   # 3.0

This works because a[0] and a[2] are digit strings, not because Python understands the complex number.

Complete conversion map

Source	Example	Result
int	float(10)	10.0
bool	float(True)	1.0
complex	float(2+3j)	TypeError
str (integer)	float("12")	12.0
str (decimal)	float("12.34")	12.34
str ("True")	float("True")	ValueError
str ("2+3j")	float("2+3j")	ValueError
Digit character	float("2+3j"[0])	2.0

Practice exercises

Exercise 1 — Convert both values and compare their types.

float(100)
float("100")

Both return 100.0, but one starts as an integer and the other as a string.

Exercise 2 — Compare these two lines:

float("12.34")
int("12.34")

Why does only the first succeed?

Exercise 3 — Predict the output:

a = "2+3j"

print(float(a[0]))
print(float(a[2]))

Expected answer: 2.0 and 3.0.

Quick knowledge check

1. What is float(10)?
   10.0

2. Does float("12.34") work?
   Yes. It converts the numeric string into a floating-point number.

3. Does float(2+3j) work?
   No. It raises TypeError because complex values are not accepted.

Summary

• float() converts possible values into floating-point numbers.
• int, bool, integer strings, and decimal strings all convert successfully.
• complex values raise TypeError.
• Non-numeric strings raise ValueError.
• Key contrast: float("12.34") is valid. int("12.34") is a ValueError.