Before diving into the simulation, recall that an RC circuit consists of a resistor (`R`) and a capacitor (`C`) connected in series. When a voltage is suddenly applied, the capacitor voltage rises gradually, following the equation:  
`Vc(t) = V * (1 - exp(-t/(R*C)))`  
where `Vc(t)` is the voltage across the capacitor at time `t`, `V` is the supply voltage, `R` is resistance, and `C` is capacitance. The time constant `τ = R*C` indicates how quickly the capacitor charges. After one time constant, the capacitor voltage reaches approximately **63%** of its final value. This property is fundamental for timing, filtering, and transient response analysis in electrical engineering.


import unittest
import user_code
import ast
import re   
import importlib
import csv
import unittest
import importlib
import io
import sys
import numpy as np
from unittest.mock import patch
import matplotlib
matplotlib.use('Agg')  # Use non-interactive backend for testing

class TestTask(unittest.TestCase):
    def test_time_constant_calculation(self):
        import user_code
        importlib.reload(user_code)
        # Patch plt.show to prevent plot window
        with patch('matplotlib.pyplot.show'):
            captured = io.StringIO()
            sys.stdout = captured
            user_code.simulate_rc_charging(1000, 10e-6, 5, 5)
            sys.stdout = sys.__stdout__
            output = captured.getvalue().strip()
        try:
            tau_printed = float(output)
        except Exception:
            tau_printed = None
        _dynamic_test(
            self,
            tau_printed is not None and abs(tau_printed - 0.01) < 1e-6,
            "Time constant is correctly calculated and printed as 0.01",
            f"Expected time constant 0.01, got {output}",
        )

    def test_voltage_curve_calculation(self):
        import user_code
        importlib.reload(user_code)
        # Patch plt.show to prevent plot window
        with patch('matplotlib.pyplot.show'):
            R, C, V, t_end = 1000, 10e-6, 5, 5
            t = np.linspace(0, t_end, 500)
            expected_vc = V * (1 - np.exp(-t / (R * C)))
            # Patch plt.plot to capture data
            with patch('matplotlib.pyplot.plot') as mock_plot:
                user_code.simulate_rc_charging(R, C, V, t_end)
                called = False
                for call in mock_plot.call_args_list:
                    args, kwargs = call
                    if len(args) >= 2:
                        t_arg, vc_arg = args[0], args[1]
                        if isinstance(t_arg, np.ndarray) and isinstance(vc_arg, np.ndarray):
                            # Check that curves match closely
                            if np.allclose(vc_arg, expected_vc, rtol=1e-3, atol=1e-3):
                                called = True
                                break
                _dynamic_test(
                    self,
                    called,
                    "Voltage curve is computed and plotted using RC charging equation.",
                    "Voltage curve was not correctly computed or plotted.",
                )

    def test_63_percent_voltage_line(self):
        import user_code
        importlib.reload(user_code)
        # Patch plt.show to prevent plot window
        with patch('matplotlib.pyplot.show'):
            V = 5
            tau_voltage = V * (1 - np.exp(-1))
            with patch('matplotlib.pyplot.axhline') as mock_axhline:
                user_code.simulate_rc_charging(1000, 10e-6, 5, 5)
                found = False
                for call in mock_axhline.call_args_list:
                    args, kwargs = call
                    if 'y' in kwargs:
                        yval = kwargs['y']
                    elif len(args) > 0:
                        yval = args[0]
                    else:
                        continue
                    if abs(yval - tau_voltage) < 1e-3:
                        found = True
                        break
                _dynamic_test(
                    self,
                    found,
                    "63% voltage level is marked on the plot.",
                    "63% voltage level was not marked on the plot.",
                )

    def test_time_constant_vertical_line(self):
        import user_code
        importlib.reload(user_code)
        # Patch plt.show to prevent plot window
        with patch('matplotlib.pyplot.show'):
            tau = 1000 * 10e-6
            with patch('matplotlib.pyplot.axvline') as mock_axvline:
                user_code.simulate_rc_charging(1000, 10e-6, 5, 5)
                found = False
                for call in mock_axvline.call_args_list:
                    args, kwargs = call
                    if 'x' in kwargs:
                        xval = kwargs['x']
                    elif len(args) > 0:
                        xval = args[0]
                    else:
                        continue
                    if abs(xval - tau) < 1e-6:
                        found = True
                        break
                _dynamic_test(
                    self,
                    found,
                    "Time constant is marked on the plot.",
                    "Time constant was not marked on the plot.",
                )

def _dynamic_test(test_case, condition, success_message, failure_message):
    if condition:
        test_case._testMethodName = success_message
        test_case.assertTrue(True, success_message)
    else:
        test_case._testMethodName = failure_message
        test_case.fail(failure_message)

def normalize_text(text):
    text = text.lower()
    text = re.sub(r"\\s{2,}", " ", text)
    text = re.sub(r"\\s*([,:?])\\s*", r"\\1 ", text)
    return text.strip()

def change_var(code: str, var_name: str, value: str) -> str:
    tree = ast.parse(code)
    lines = code.splitlines()
    changed = False
    # Collect all assignment nodes to modify
    assign_nodes = [
        (i, node)
        for i, node in enumerate(tree.body)
        if isinstance(node, ast.Assign)
        and any(isinstance(target, ast.Name) and target.id == var_name for target in node.targets)
    ]

    # If nothing to change, return unmodified code
    if not assign_nodes:
        return code

    # Perform replacements for all matching assignments (from last to first to not break line offsets)
    for i, node in reversed(assign_nodes):
        start_line = node.lineno - 1
        line = lines[start_line]
        indent = ' ' * (len(line) - len(line.lstrip()))
        lines[start_line] = f"{indent}{var_name} = {value}"
        next_line = len(lines)
        for next_node in tree.body[i+1:]:
            if hasattr(next_node, 'lineno'):
                next_line = next_node.lineno - 1
                break
        if next_line > start_line + 1:
            lines[start_line+1:next_line] = []
        changed = True

    return '\\n'.join(lines) if changed else code

if __name__ == "__main__":
    unittest.main()


test_main.py

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Challenge: Simulate an RC Charging Circuit

Solution

Challenge: Simulate an RC Charging Circuit

Solution