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io.open() is an alias for for the builtin open() function: https://docs.python.org/3/library/io.html#io.open If open() cannot open a file, an OSError is raised: https://docs.python.org/3/library/functions.html#open EnvironmentError is kept for compatibility with previous versions; starting from Python 3.3, it is an alias of OSError: https://docs.python.org/3/library/exceptions.html#EnvironmentError Directly yield from an iterable instead of iterating to yield items.