Tag: Python
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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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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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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.
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__new__ vs __init__: Why Python constructors rarely need __new__
Heard about __new__, the constructor in Python? You can basically forget about it. It’s for advanced, unusual cases. Just use __init__! It runs automatically on every new instance. __init__ is for assigning attributes to self (what other languages call “instance variables”).
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uv python pin: Changing the Python version in .python-version
Using uv? The Python version is stored in .python-version. Change it with But: uv gives an error: 3.13 is incompatible with requires-python in pyproject.toml of ‘>=3.14.’
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Unique values from a Python list: Using set() and unpacking
Want to get the unique values from a Python list? Use a set! Of course, list elements must be hashable!
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Sorting a Python dict by value with operator.itemgetter
Sorting a Python dict by value? Instead of: Use itemgetter. It’s cleaner AND ~25% faster: Sort by value, then by key:
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Python file modes: Use ‘x’ instead of ‘w’ to avoid overwriting
Write to a file in Python with open(filename, ‘w’). What if filename already exists? Its contents are gone. (I hope you have backups!) Instead, try: If filename already exists, ‘x’ raises an exception. For new files, ‘x’ and ‘w’ are the same.
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Python line continuation: How parentheses split long lines
Use parentheses to split long Python code across lines: But with parentheses, Python sees it as one line: Or in comprehensions…