Expfit: Fit exponentials to noisy time series

Expfit is a lightweight Python package to fit exponentials to noisy time series, typically with the aim of extracting time constants.

It aims to meet the following goals:

• Fit scaled and vertically transposed exponentials y = a + b * exp(c * x) without requiring initial parameter estimates.
• Fit double, triple, and quadruple exponentials y = a + b_i * exp(c_i * x) where each exponential term is decaying (c_i < 0) and all b_i have the same sign.
• Fit multiple decaying exponentials in data with multiple b_i signs.
• Be lightweight: use good initial strategies and properties of exponentials to simplify the optimisation problem.

Although a relatively simple task, expfit has unit tests, and reported failures will be added to its test suite to create a reliable tool for this sometimes fiddly operation.

State 2026-04-10

Does single exponentials well

Building on initial guesses for single, also started adding double which it does OK