Python Practice 19/22

Fix the bug: the copy that everyone shares

Every new account starts from one settings template, and each account gets its own copy so people can change their own settings without touching anyone else's. That is exactly what .copy() is for, and the code below calls it.

Run it. Ana goes dark, mutes billing, and turns off email. Ben was created one line after her and has not been touched since. Read Ben's line anyway: he is muting billing and his email is off. Read the template's line: it is muting billing too. Nothing raised. Nothing warned. Every account created from here on starts with Ana's preferences baked in.

Now look at what did not leak. Ana's theme is dark, Ben's is still light, the template's is still light. The copy is not fake - it is real, and it is exactly one level deep. That is what keeps this bug alive for months: someone tests the copy by changing a top-level value, watches the two dicts disagree, concludes copying works, and ships it.

Python could have made every copy go all the way down. It does not, because most copies do not need to - duplicating every nested object in a large structure costs real time and memory, and some objects (an open file, a database connection, a lock) cannot be meaningfully duplicated at all. So the cheap copy is the one you get by default, and the thorough one is a deliberate import away.

Your task: fix new_account so each account is fully independent of the template and of every other account, at any depth. Ana's changes must show up on Ana's line only. Ben and the template must both still read light, [], {'email': True, 'sms': False}.

You'll practice:

Reading a plausible output as a claim, then checking it against what you already know is true
Spotting where a shallow copy leaves its nested objects shared, and copying deeply instead

# Each new account gets its own copy of the template. Run it and read all three
# lines - Ben never touched anything, and the template is supposed to be a template.
DEFAULTS = {
    "theme": "light",
    "muted_channels": [],
    "notifications": {"email": True, "sms": False},
}

def new_account(defaults):
    return defaults.copy()

ana = new_account(DEFAULTS)
ben = new_account(DEFAULTS)

ana["theme"] = "dark"
ana["muted_channels"].append("billing")
ana["notifications"]["email"] = False

print("ana:     ", ana["theme"], ana["muted_channels"], ana["notifications"])
print("ben:     ", ben["theme"], ben["muted_channels"], ben["notifications"])
print("template:", DEFAULTS["theme"], DEFAULTS["muted_channels"], DEFAULTS["notifications"])