Category: Python
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Python dict insertion order: What happens when you re-add a key
Modern Python dicts remember insertion order: This applies in for loops, too!
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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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Python str.split and rsplit: Splitting strings with maxsplit
Break a Python string into a list of strings with str.split: Limit splits with maxsplit: Or split from the right, with a limit:
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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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Pandas columns overlap error: Fixing join with lsuffix and rsuffix
Joining Python Pandas DataFrames with overlapping columns? You’ll hit: ValueError: columns overlap but no suffix specified Fix with lsuffix and rsuffix: Now the columns are: x_left, x_right, y_left, y_right Problem solved!
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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.