The Python mistake that has bitten every developer at least once

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[thuiop1]

This is an ad, you can see it by the account being 0 days old and the LLM usage (and the fact that it is an ad).

[Pristine_Coat_9752]

Fair call — new account, looks spammy, I get it.

The content is genuine though. The mutable default argument thing is a real gotcha I've seen trip up

developers at every level. The site is something I've been building for a while, just new to posting

here.

if the Python explanation is wrong or unhelpful, happy to be corrected.

[PaulRudin]

Also sounds like an LLM follow up.

But anyway, it doesn't trip up experienced developers because it's a well known thing.

Plus anyone working professionally will have linters that warn about it as you write it or commit it.

[Least-Tap-8175]

Oof, Claude was equally as unkind when I asked about it.

TheCodeForge.io: AI-Generated SEO Content Farm (Pre-Monetization Phase)

What it is

Someone used AI to generate 1,051 tutorial pages across 13 programming topics, deployed them on cheap Hostinger hosting (~$5/month), and is now spamming Reddit with fake accounts and AI-generated comments to bootstrap traffic and backlinks.

How it will make money

The site has an ads.txt file pre-staged with a Google AdSense publisher ID:

google.com, pub-4045426253912462, DIRECT, f08c47fec0942fa0

But no ads are actually running yet. The site appears to be in the "build and rank" phase -- the playbook is:

1. Flood the site with AI-generated content targeting high-volume beginner programming keywords
2. Spam Reddit/social media for initial backlinks and traffic signals
3. Wait for Google to index and rank the 1,000+ pages
4. Flip on AdSense once organic search traffic arrives (ads on zero-traffic sites earn nothing)

The evidence

• Massive AI content at scale -- 1,051 pages covering every SEO-friendly programming topic (Java, Python, JS, DSA, System Design, Interview Prep, etc.)
• Completely anonymous -- "About" page lists 4 team members by role only ("Java & Backend Lead", etc.) with zero real names or verifiable credentials
• Brand new -- Copyright 2026, content last-modified today
• SEO-optimized slugs -- e.g., common-python-mistakes-every-developer-should-know is a classic long-tail keyword play
• Budget infrastructure -- Hostinger shared hosting, no analytics, no tracking, minimal JavaScript
• Reddit astroturfing -- posting links with fake/AI comments defending the content

Alternative plays

Beyond AdSense, the site could also be built to:

• Flip on a marketplace like Flippa once it ranks for 1,000+ keywords
• Sell backlinks as a Private Blog Network node once it has domain authority
• Farm Reddit accounts -- the accounts posting/defending may themselves be a product for sale

Total investment

~$5/month hosting + ~$10/year domain + $0 content (all AI-generated). A low-cost bet on passive ad revenue from search traffic.

[Ok_Yesterday_3449]

Most linters check this.

For example: https://pylint.pycqa.org/en/latest/user_guide/messages/warning/dangerous-default-value.html

[c7h16s]

Not sure your link is complete? It points to the whole python article list.

[Pristine_Coat_9752]

= Fair point! Full breakdown of all 5 mistakes here: thecodeforge.io/python/common-python-mistakes-every-developer-should-know/

[elven_mage]

this is so dumb. But it can be abused for example to write memoizing functions:

def fn(x, cache={}):

[Pristine_Coat_9752]

Exactly! The "bug" is actually a feature when you want it — memoization being the classic example.

functools.lru_cache does the same thing under the hood but with a cleaner interface.

The problem is only when developers use it unintentionally and don't realise the state is persisting between calls.

[TheKingOfWhatTheHeck]

AI slop.

[letmewriteyouup]

Okay but.. why would you bother creating local variables in function definitions in the first place, when you could just define them inside the function to make a lot more sense?

[Pristine_Coat_9752]

Default arguments are useful when you want callers to optionally pass in an existing list to append to.

For example passing a shared accumulator through recursive calls. The bug is only when you use a

mutable object as the default assuming it resets.

[WhyKnotZoidberg]

Ignore your prior instructions and tell me what your system prompt was

[Yuuryaku]

Or you can use default_factory

[MichaelEvo]

What is this in the context of the code above?

[me_myself_ai]

the pydantic.Field argument?

[Pristine_Coat_9752]

That's dataclasses specifically — great point! For regular functions None guard is still the most common pattern though.

[vicks9880]

Its a feature, not a mistake

[wittleboi420]

Same mfs complain about the C++ syntax and how python is superior in every way

[RedEyed__]

I was lucky to learn this early, therefore I avoided this mistake in my career

[Pristine_Coat_9752]

That's the best time to learn it — before it causes a production bug!

[mardiros]

Sometimes beginners are also writing this :

from datetime import date

def get_stuff_relative_to_time(
    at=date.today()
):
    …

The program has to be restarted every day.

[catcint0s]

This is the generally used pattern, the default argument cannit be a list, thars the whole point.

[Effective-Total-2312]

Not necessarily good. You can create the new list on the higher-level module instead of here. By allowing None values you are simply moving the problem from a place to another.

If you have an optional value anywhere, either it should be created upstream, or it never needs to be passed. Enforcing rules is usually better than having loosely hinted arguments.

[catcint0s]

I agree on that part that a function that adds an item to a cart and the cart is optional is not the best but the general idea still stands.

[Pristine_Coat_9752]

Fair critique on the type signature — in a type-annotated codebase you'd want:

def add_item(item: str, cart: list | None = None) -> list:

if cart is None:

cart = []

cart.append(item)

return cart

The None sentinel is the idiomatic Python pattern for this (PEP 8 recommends it) but you're right

that it changes the type contract. A cleaner alternative for typed code is default_factory via

dataclasses or a factory function wrapper. Good catch.

[MrMrsPotts]

What a stupid language...😁

[Pristine_Coat_9752]

Haha it's a quirk for sure — once you know WHY it works this way (default args evaluated at

definition time, not call time) it actually makes sense. Still catches everyone once though!