### Secant Method for XIRR Calculation (Fallback)

Source: https://github.com/peliot/xirr-and-xnpv/blob/master/README.rst

Provides a fallback implementation of the secant method for calculating XIRR when SciPy is not available. This method approximates the IRR but may not fail gracefully if no answer is found. It requires a list of cash flows and their corresponding dates.

```python
from typing import List, Union
from datetime import date

def secant_method(f, x1, x2, tol=1e-8, max_iter=100):
    """
    Find a root of the function f using the secant method.

    Arguments:
    f -- the function to find the root of
    x1, x2 -- initial guesses
    tol -- tolerance for convergence
    max_iter -- maximum number of iterations

    Returns:
    the approximate root
    """
    for _ in range(max_iter):
        if abs(x2 - x1) < tol:
            return x2
        f1 = f(x1)
        f2 = f(x2)
        if abs(f2) < tol:
            return x2
        x = x2 - f2 * (x2 - x1) / (f2 - f1)
        x1, x2 = x2, x
    raise RuntimeError("Secant method did not converge")

def xirr_secant(cashflows: List[tuple[date, Union[int, float]]], guess: float = 0.1) -> float:
    """
    Calculate the Internal Rate of Return for a series of cash flows occurring at
    irregular intervals using the secant method.

    Arguments:
    cashflows -- a list of tuples, where each tuple is (date, amount)
    guess -- initial guess for the rate

    Returns:
    the Internal Rate of Return
    """
    if not cashflows:
        raise ValueError("cashflows cannot be empty")

    # Use a second guess slightly different from the first
    guess2 = guess * 1.01 if guess != 0 else 0.01

    return secant_method(lambda r: xnpv(r, cashflows), guess, guess2)
```

--------------------------------

### Calculate XNPV using Python

Source: https://github.com/peliot/xirr-and-xnpv/blob/master/README.rst

Calculates the Net Present Value (NPV) for a series of cash flows occurring at irregular intervals. Requires a discount rate, a list of cash flows, and their corresponding dates. This implementation relies on the SciPy library.

```python
from typing import List, Union
from datetime import date

def xnpv(rate: float, cashflows: List[tuple[date, Union[int, float]]]) -> float:
    
```

--------------------------------

### Calculate XIRR using Python with SciPy

Source: https://github.com/peliot/xirr-and-xnpv/blob/master/README.rst

Calculates the Internal Rate of Return (IRR) for a series of cash flows occurring at irregular intervals. This function uses the numerical solver from the SciPy library. It requires a list of cash flows and their corresponding dates. The accuracy depends on the solver's convergence.

```python
from typing import List, Union
from datetime import date
from scipy.optimize import newton

def xirr_scipy(cashflows: List[tuple[date, Union[int, float]]], guess: float = 0.1) -> float:
    
```

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