Python csv: 14 flashcards
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1. What does the `csv` module provide for reading CSV files?
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The `csv` module provides `csv.reader`, which returns an iterator that yields each row as a list of strings.
2. Why should you open files with `newline=''` when using the csv module?
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To prevent extra blank lines from appearing in the output, especially on Windows, because the csv module handles line endings itself.
3. What is the purpose of `csv.DictReader`?
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It reads CSV rows as dictionaries, using the header row as keys, so you can access columns by name.
4. How does the csv module handle fields that contain commas, quotes, or newlines?
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It automatically wraps such fields in double quotes and escapes internal quotes by doubling them.
5. What is a dialect in the context of the csv module?
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A dialect is a set of formatting parameters (like delimiter, quotechar, and quoting) that defines a specific CSV variation, which can be registered and reused.
6. How can you read a CSV from a string instead of a file?
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Wrap the string with `io.StringIO` to create a file-like object, then pass it to `csv.reader`.
7. What data type does the csv module return for each field when reading?
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All fields are returned as strings; you must explicitly convert to numbers or other types as needed.
8. What method do you use to write a single row with `csv.writer`?
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`writerow()` takes an iterable of values and writes it as a single line in the CSV file.
9. How do you specify a semicolon as the delimiter when reading a CSV file?
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Pass the `delimiter=';'` parameter to `csv.reader` (or `csv.writer`).
10. What is the main advantage of using the csv module over manual string splitting?
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It correctly handles quoting and escaping of fields that contain commas, quotes, or newlines, which manual splitting would break.
11. What does `csv.writer` expect as the first argument?
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A file object opened for writing (with `newline=''`), such as the result of `open('file.csv', 'w', newline='')`.
12. How does `csv.DictWriter` determine the column order when writing?
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You must provide a `fieldnames` list that defines the order of columns; it uses that list to write the header and map dictionary keys.
13. What is the default quoting behavior of the csv module?
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The default is `QUOTE_MINIMAL`, which quotes fields only when necessary (e.g., when they contain the delimiter, quotechar, or newline).
14. Why is the csv module considered a foundation for data processing?
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It provides a reliable way to read and write tabular data, which is often the first step before using higher-level libraries like pandas.
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