Benford's Law Forensic Accounting & Invoice Fraud Detector in Python

import numpy as np
import pandas as pd
from scipy.stats import chisquare

def benford_forensic_audit(amount_series: pd.Series):
    clean = amount_series.dropna().abs()
    clean = clean[clean >= 10]
    first_digits = clean.astype(str).str.lstrip('0.').str[0].astype(int)
    first_digits = first_digits[first_digits.between(1, 9)]
    
    obs_counts = first_digits.value_counts().reindex(range(1, 10), fill_value=0)
    obs_pct = obs_counts / len(first_digits)
    benford_pct = pd.Series([np.log10(1 + 1/d) for d in range(1, 10)], index=range(1, 10))
    
    stat, p_val = chisquare(obs_counts, f_exp=benford_pct * len(first_digits))
    return {'Chi2_Stat': round(stat, 2), 'P_Value': round(p_val, 4), 'Suspicion_Flag': p_val < 0.05}