TheAlgorithms/Python · error · ValueError

differentiate() requires a float as input for position

Error message

differentiate() requires a float as input for position

What it means

Raised by differentiate() in maths/dual_number_automatic_differentiation.py when position is neither float nor int. The evaluation point becomes the real part of the seed Dual(position, 1), so it must be a real number; strings and other types fail the isinstance(position, (float, int)) check and raise ValueError with this message (note the message says 'float' even though ints are accepted).

Source

Thrown at maths/dual_number_automatic_differentiation.py:121

    >>> differentiate(lambda y: y ** 2, 4, 3)
    0
    >>> differentiate(8, 8, 8)
    Traceback (most recent call last):
        ...
    ValueError: differentiate() requires a function as input for func
    >>> differentiate(lambda x: x **2, "", 1)
    Traceback (most recent call last):
        ...
    ValueError: differentiate() requires a float as input for position
    >>> differentiate(lambda x: x**2, 3, "")
    Traceback (most recent call last):
        ...
    ValueError: differentiate() requires an int as input for order
    """
    if not callable(func):
        raise ValueError("differentiate() requires a function as input for func")
    if not isinstance(position, (float, int)):
        raise ValueError("differentiate() requires a float as input for position")
    if not isinstance(order, int):
        raise ValueError("differentiate() requires an int as input for order")
    d = Dual(position, 1)
    result = func(d)
    if order == 0:
        return result.real
    return result.duals[order - 1] * factorial(order)


if __name__ == "__main__":
    import doctest

    doctest.testmod()

    def f(y):
        return y**2 * y**4

    print(differentiate(f, 9, 2))

View on GitHub (pinned to f5988cc097)

Solutions

1. Convert before calling: differentiate(f, float(position), 1).
2. Validate numeric input at your boundary (reject empty strings, parse with float()).
3. Cast Decimal/Fraction positions to float explicitly, accepting the precision change.

Example fix

# before
differentiate(lambda x: x ** 2, input('x: '), 1)  # str -> ValueError

# after
x0 = float(input('x: '))
differentiate(lambda x: x ** 2, x0, 1)

if not isinstance(position, (float, int)) or isinstance(position, bool):
    position = float(position)  # after your own parse validation

def is_real_number(v) -> bool:
    return isinstance(v, (int, float)) and not isinstance(v, bool)

try:
    d = differentiate(func, position, order)
except ValueError as e:
    if 'float as input for position' in str(e):
        d = differentiate(func, float(position), order)
    else:
        raise

When it happens

Trigger: Calling differentiate(lambda x: x**2, '', 1), differentiate(f, None, 1), or differentiate(f, '3.0', 1). Note differentiate(lambda x: x**2, 3, 1) is fine — int passes.

Common situations: Position read as a string from argv, input(), or a config file; None from an optional parameter default; Decimal or Fraction position values that are not float/int subclasses.

Related errors

• differentiate() requires a function as input for func
• differentiate() requires an int as input for order
• Both points must have the same dimension.
• Monogons and Digons are not polygons in the Euclidean space
• All values must be greater than 0

AI-assisted analysis of TheAlgorithms/Python@f5988cc097 (2026-08-14). Data as JSON: /api/errors/daf451eee39defe5. Report an issue: GitHub.