Pretend that you have a number that is always pointing somewhere in your temporary file. It starts a 0. If you then write "lalalala" (8 characters) it will point after these at position 8, so that you can write more stuff after your previous text later by calling write.
The read method reads all the data from the current position of the "pointer" to the end of the file. If you simply call it after writing, you wont get anything since you are already at the end of the file. Therefore you "rewind" the pointer back to the start by calling seek which takes the position in the file to go to... your_file.seek(0) returns you to the start of the file to read all of its data - Björn Kempén
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