Common Bad Practices in Python and How to Avoid Them

Many articles teach how to write good Python code, but this piece focuses on the opposite: common bad habits that can hide bugs or make code harder to maintain. The examples are written for Python 3.6.

1. Using mutable objects as default function arguments – The article shows a function that unintentionally shares a list across calls, leading to unexpected results. Replacing the mutable default with None and initializing inside the function resolves the issue.

2. File handling – Beginners often open files without using a context manager, forgetting to close them. The article recommends the with open(...) pattern, which automatically handles resource cleanup and results in cleaner code.

3. Catching all exceptions – Using a bare except Exception can swallow unrelated errors such as KeyboardInterrupt or AssertionError . Instead, catch specific exceptions like ValueError , AttributeError , or TypeError and handle them appropriately.

4. Ignoring Python's for…else syntax – The article explains that the else block runs only when the loop finishes without a break . It demonstrates a more idiomatic way to check for the presence of an odd number in a list using this construct.

5. Iterating over dictionary keys – Instead of looping over dict.items() and then accessing the key, the article shows that iterating directly over the dictionary yields the keys, making the code clearer and more concise.

These points reflect personal preferences rather than strict rules; the goal is to choose the most suitable practice for each situation.