Phase 0 · ToolsWorking with data

Files and CSV data

About 25 minLooking ahead: some prerequisites aren't done yet

Why this matters

Real data lives in files. Think of CSV exports from ad platforms and CRMs, documents you want a model to summarize, example prompts and replies for testing, and logs of everything your app sent to a model. Before you can analyze, embed or train on any of it, you need to get it into Python. Then you need to write your results back out.

Write text to a file and read it back with open() and with
Process a file one line at a time
Read a CSV export into rows you can calculate with, and write one
Build file paths with pathlib

Writing and reading a file

The academy runs Python inside your browser, with its own small file system. Files you create here live in this sandbox, not on your computer. But the code is exactly what you'd run on your own machine.

Working with a file is like using a notebook: open it, read or write, then close it.

with open("notes.txt", "w") as f:
    f.write("Q4 plan\n")
    f.write("Raise search budget\n")
    f.write("Test new subject lines\n")

with open("notes.txt") as f:
    contents = f.read()

print(contents)

open(path, mode) opens a file and gives you a file object, here named f. The mode says what you'll do with it:

Mode	Meaning
`"r"`	read (the default if you leave the mode out)
`"w"`	write: creates the file, or erases an existing one first
`"a"`	append: adds to the end of the file

write doesn't add line breaks for you. \n inside a string is the newline character. It marks the end of a line.

with ... as f: makes sure the file is closed when the indented block ends, even if an error happens partway through. Always open files this way. A file left open can lose data that was written but not yet saved.

Tip: add encoding="utf-8" when you open text files on your own computer, for example open("notes.txt", encoding="utf-8"). Then accents, non-English text and emoji read correctly on every system.

One line at a time

Looping over a file object gives you its lines one by one. Each line still ends with its \n, so .strip() it:

with open("subjects.txt", "w") as f:
    f.write("Last chance: 30% off\n")
    f.write("New arrivals you'll love\n")
    f.write("Your weekly picks\n")

with open("subjects.txt") as f:
    for line in f:
        line = line.strip()
        print(len(line), line)

f.readlines() gives you all the lines as a list instead. Looping is better for big files, because Python only holds one line in memory at a time. A 2 GB log file is fine to loop over. It would be painful to load all at once.

Appending is how logs grow:

with open("log.txt", "w") as f:
    f.write("batch 1 sent\n")

with open("log.txt", "a") as f:
    f.write("batch 2 sent\n")

with open("log.txt") as f:
    print(f.read())