Explain CSV code

What this CSV sample shows

header and rows

The first line names the columns, and every other row gives one value per column, separated by commas. Python's csv.DictReader used the header as keys and read 6 orders, each with order_id, customer, item, qty, price and note.

quotes

A value with a comma in it goes in double quotes, so the comma is part of the value. "Lee, Sam" came out as Lee, Sam and "Mug, large" as Mug, large. Inside quotes, two double quotes stand for one: "said ""no rush""" came out as said "no rush".

line breaks in a value

A quoted value can hold a line break. Order 1004's note runs over two lines of the file, and the reader kept it as one value, leave at and back door with a line break between, so the file has 8 lines but 6 orders.

empty values and text

A trailing comma means an empty last value: orders 1001, 1005 and 1006 came out with a note of ''. Every value is text, quantities and prices too, so code has to convert them: int(qty) * float(price) over all rows gave 154.5.

Good to know

• Read with the csv module in Python 3.14, opening the file with newline="" as the module's docs ask.
• Not every CSV uses commas. A file separated by ; came back as one column, ['id;name'], until it was read with delimiter=";".
• A space after a comma counts as part of the value: a, "b, c" gave ' "b' and ' c"', while skipinitialspace=True read it as b, c.
• Writing with csv.writer quoted Lee, Sam and doubled the quotes in said "hi" by itself, and ended the row with \r\n.

Watch out: Splitting lines on commas breaks on quoted values. line.split(",") on the Lee, Sam row gave 7 pieces for 6 columns, with "Lee and Sam" apart, and reading line by line would cut order 1004's note in two. Use a CSV reader instead of split.