After a block-opening statement, the next line is indented by 4 spaces (in the Python Shell window by one tab). After certain keywords (break, return etc.) the next line is dedented. In leading indentation, Backspace deletes up to 4 spaces if they are there. Tab inserts 1-4 spaces (in the Python Shell window one tab). See also the indent/dedent region commands in the edit menu.
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