Code Review and Refactor: Summing a CSV Column

Context

You are reviewing a short Python script that sums a numeric column from a CSV using pandas. Your tasks are to identify problems, refactor into a small, testable module, add tests, define an environment, and explain design choices and complexity trade-offs.

Given Script

# script.py
import pandas as pd
DATA_PATH = 'data.csv'
result = None

def compute_total(col):
    df = pd.read_csv(DATA_PATH)
    total = 0
    for x in df[col]:
        if x == '':
            total += 0
        else:
            total += float(x)
    print(total)

compute_total('amount')

Tasks

1. Identify at least five defects or risks (correctness, performance, readability, resource management, security).
2. Refactor into a small, testable module with clear interfaces, type hints, and no global state; include input validation and assert-based precondition checks. Explain when assertions vs. exceptions are appropriate.
3. Write three pytest-style unit tests using assert statements that cover:
   • Normal inputs
   • Missing/NaN inputs
   • Malformed inputs
4. Provide an environment.yml for a Conda environment (Python 3.11, pinned dependencies) and the exact commands to create/activate it.
5. Explain the benefits of modularization for maintainability, dependency management, and testability.
6. State the time/space complexity before and after refactoring and any I/O bottlenecks you would address.