Tag: Python
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Python defaultdict: Pass a factory function, not a value
Using Python defaultdict? Don’t pass a value: Pass a function that returns the default:
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Python positional-only parameters: Fixing **kwargs conflicts
Your Python function takes a filename and **kwargs. But this raises an error, since “filename” is passed twice: Solution: Set filename to be positional only: Now any keyword argument works!
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%xmode in IPython: Shortening long Python tracebacks
Tired of super-long Python Pandas backtraces in Jupyter or IPython? Use the %xmode magic method: %xmode Minimal Now you just see the error, not 50 lines of library internals. You can also set it to Plain, Context, or Verbose.
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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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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.