Merging CSV files with mismatched column orders using Python's standard library

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Combining monthly CSV exports becomes tricky when files have different column orders, extra columns, or a byte order mark added by Excel. The article presents a short standard-library Python script that handles all three issues and adds a column recording each row's source file. It relies on DictReader to match values by header name, reads files with the utf-8-sig encoding to strip the BOM, and builds the output header from the union of all columns in first-seen order. An example with two files shows the merged output with blanks filled in and source tracking included. The author notes the approach loads everything into memory, so it fits files up to a few hundred megabytes, and assumes comma-separated UTF-8 input unless the delimiter is changed.

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