Category: Python
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Modifying a Python list while iterating: Only append is safe
Think you can’t modify a Python list while iterating? You can append to it: This prints 10, 20, 30, 100, 400, and 900. BUT don’t insert or remove. That’ll cause real trouble.
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Python multiple return values: Returning and unpacking tuples
Think Python functions can return multiple values? They actually return ONE value, a tuple: Unpack it into separate variables:
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__setitem__ in Python: Custom logic for square-bracket assignment
Let users assign using [] in Python, with custom logic with __setitem__: sl.d[‘2’, ‘4’, ’99’, ‘8’]
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Python `__getitem__`: Custom logic behind square brackets
The __getitem__ method in Python can do more than simple lookups: Use [] for custom logic — returning filenames, random numbers, or database records.
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Python __getitem__: Adding bracket notation to your classes
Want your Python object to support bracket notation []? Define __getitem__:
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Pandas join: Why you need a one-column DataFrame, not a Series
Python Pandas gotcha: You can’t join a single column! Why? join() is a data frame method. A single column is a Series. Solution: Use [[ ]] to get a one-column data frame:
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Pandas currency strings: Strip USD, then astype(float) and sum
I used Python Pandas to total AWS S3 payments. Data was in a CSV file. But numbers in the “Amount” column were written as “USD 12.34”. Here’s what I did:
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Python method calls: Why dict.items needs parentheses
Python gotcha: Forgetting () on method calls! Result: Error: ‘builtin_function_or_method’ object is not iterable Remember: () calls the method!
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Pandas 3.0 pd.col: Replacing lambda in loc and assign
Python Pandas 3.0 introduces pd.col, which shortens queries: Also:
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Python from import doesn’t save memory: The module still loads
Think “from modname import funcname” saves memory in Python? It doesn’t! (Lots of people think it does.) The full module still loads (see sys.modules). The only difference: “modname” isn’t in your namespace, just “funcname”. “from import” is convenient. It doesn’t save memory.