Week 10: Files and Strings

🎯 Objectives

Until now, everything your programs knew came from the code itself or from someone typing at the keyboard, and everything they found out vanished when the program ended. This week your programs start reading and writing files, so the data can live somewhere other than your code and the results can stick around.

Text from a file is rarely in the shape you want it, though. A line ends with an invisible \n, has stray spaces, and holds several pieces of information in one string. So the other half of this week is the toolkit for taking a string apart: slicing and string methods like strip() and split(). Along the way you'll meet an idea that explains a lot of puzzling bugs: a string can never be changed, but a list can, even from inside a function.

By the end of the week you should be able to:

• Write lines to a new text file, append lines to an existing one, and read a file one line at a time with a with statement.
• Slice a string or a list, with and without the start, stop, and step, and use string methods such as strip(), split(), join(), startswith(), and the case methods (upper(), lower()) to pull apart and transform text from a file.
• Explain why a string method returns a new string instead of changing the original, and predict what happens to a list that a function changes in place.

✅ Must do

These three are the core of the week. Do them first.

1. The in-class activities, including the Reading/Writing Files activity. If you missed a class, work through its handout on your own.
2. The reading: one of the options in "Read about it," below.
3. PA3 Parts C and D, which read your program's data from a file and write its results to another. Submit the whole PA to Gradescope when it's done.

📚 Study menu (should do)

Class covered reading and writing files, slicing and string methods, and how strings and lists differ when you try to change them. Everything below is optional, but each item trains something Quiz 4 or PA3 needs. Pick what you need.

If you only have an hour

1. Twenty minutes. "See what's really in a line," below, in Thonny.
2. Ten minutes. "Slice it two ways," on paper first.
3. Thirty minutes. "Rehearse a function and its test," with a timer running.

Read about it

Pick one option.

Python for Everybody: 8. Files and 7. Strings

• 8.1 Persistence, 8.2 Opening files, 8.3 Text files and lines, 8.4 Reading files, 8.5 Searching through a file, and 8.8 Writing files.
• 7.4 String slices, 7.5 Strings are immutable, 7.9 String methods, and 7.10 Parsing strings.

W3Schools: the reference-style skim

• Python Strings, with its Slicing Strings, Modify Strings, and String Methods subpages.
• Python File Handling, Python Read Files, and Python Write/Create Files.

See what's really in a line

The activity in class used repr() to make the invisible parts of a line visible. Take it one step further. Type this into Thonny, but before you run it, write down what you think each print shows.

with open("scores.txt", "w") as file:
    file.write("Ana, 92\n")
    file.write("\n")
    file.write("  Ben ,85  \n")

with open("scores.txt", "a") as file:
    file.write("Cal, 78\n")

with open("scores.txt") as file:
    for line in file:
        print(repr(line))
        print(repr(line.strip()), line.strip().split(","))

Then run the program a second time. Does scores.txt get longer? Why or why not?

The rules you should be able to state afterward:

• A line from a file ends with \n (except maybe the last one). Strip it before you compare it to anything.
• "w" replaces the file, and "a" adds to the end of it.
• Strip the line, split it, and then strip each piece, because strip() only ever touches the two ends of a string.

Slice it two ways

On paper, predict each slice, with word = "Madison" and nums = [10, 20, 30, 40, 50]. Then check yourself in the Shell.

Last, compare list(range(1, 7, 2)) with "abcdefg"[1:7:2].

What this should leave you with:

• A slice works the same way on a string and on a list. Only the kind of thing you get back differs.
• [start:stop:step] follows the same rules as range(start, stop, step): start is included, stop is not.
• A slice never raises an IndexError. word[2:100] just stops at the end, and word[4:2] is empty. word[100] is still an error.

Who changed my list?

Trace this program on paper and write down each line it prints.

def shout(word):
    word = word.upper()
    return word


def add_cheer(items):
    items.append("hooray")


def reset(items):
    items = []


name = "pat"
shout(name)
print(name)
loud = shout(name)
print(loud)

team = ["go", "dukes"]
add_cheer(team)
print(team)
reset(team)
print(team)

line = "  hello  "
line.strip()
print(repr(line))

Step through it in Python Tutor if any answer surprised you, and watch the arrows: team and items point at the same list.

Look it up

Nobody memorizes every string method. You do need to know where to find them.

• In the Shell, try help(str.split) and help(str.strip). What does split do when you give it no argument at all?
• Skim the official list of string methods in the Python docs. Find two methods you haven't used yet, and try each one on a string in the Shell.
• dir("") lists every method a string has. Which ones start with is?

Rehearse a function and its test

Quiz 4 asks you to write a function and a test function, so practice the mechanics now, while nothing is at stake. In a new folder, make two files. Write the function in practice.py:

"""Practice for Quiz 4.

Name: Your Name
"""


def initials(full_name):
    """Return the capitalized first letter of each word, Ex "ada lovelace" -> "AL"."""

and its tests in test_practice.py:

import pytest
from practice import initials


def test_initials():
    assert initials("ada lovelace") == "AL"


if __name__ == "__main__":
    pytest.main()

Set a timer for fifteen minutes and fill in the function's body without looking anything up. Then press Run in Thonny on the test file, so the pytest.main() block runs your tests. Add at least two more assert lines that could catch a bug, like a name with extra spaces or with only one word. For a second round, try count_long(words, n), which counts the strings in a list longer than n characters, and write its tests before its body.

Try an AI tutor

AI is a good drill partner for slicing and string methods, as long as it hands you the questions and not the answers. It also loves shortcuts this course hasn't covered, so tell it up front what you know.

"I am a CS1 student who knows for and while loops, lists, sets, slicing, and the string methods strip, split, join, upper, and lower. Give me eight slicing and string-method expressions on one short string, one at a time. Wait for my prediction before telling me if I'm right."

"Here is my function for reading a file. Don't rewrite it. Tell me which line would break on a blank line in the file, and let me fix it."

The thing to be skeptical about this week is shortcuts. AI tends to reach for readlines(), try and except, and one-line comprehensions. They aren't wrong, but you can't use what you can't explain on Quiz 4, and a loop over the file is easier to trace by hand.

🔭 Explore more

Finished early, or just curious? None of these are required.

• Download a book in plain text from Project Gutenberg and write a program that finds its longest line, or counts the lines that mention a word you choose.
• Write a tiny "mail merge": read names from one file and write a personalized greeting for each person to another.
• Find out what str.partition, str.center, and str.title do, and when title() gives a surprising answer (try "don't stop".title()).
• Look ahead: a list can be sliced, but can you assign to a slice? Try nums[1:3] = [0] and explain what happened.

✍️ Reflection

1. What did you do this week? (Check all that apply.)

• Worked through the in-class activities
• Read the sections listed for one of the textbooks
• Finished PA3 Parts C and D
• Predicted the output in "See what's really in a line"
• Filled in the "Slice it two ways" table on paper
• Traced "Who changed my list?"
• Looked up string methods with help() or the Python docs
• Rehearsed a function and its test for Quiz 4
• Asked an AI tutor to drill me
• Tried something from "Explore more"
• Something else (tell us in the short answers)

2. Roughly how much time did you spend on CS 149 outside of class this week?

• Under 1 hour
• 1–2 hours
• 3–4 hours
• 5–6 hours
• 7+ hours

3. How ready do you feel to write a function and its test for Quiz 4?

• Ready: I can write both from a blank file without looking anything up
• Mostly: I can write the function, but the test file still trips me up
• Not yet: I need my notes or the PA to get started
• I haven't tried it yet

4. How did you test your program? Before you submitted PA3 Parts C and D to Gradescope, what files or calls did you try, and what could each one have caught? Did you try a file with a blank line, a comment line, or a line with extra spaces?

5. Give one string or file operation this week that did not do what you expected. It could be a slice that came out one character off, a split that left spaces behind, a method whose result you forgot to save, or a file that came out empty or doubled. Say what you expected, what happened, and the rule you were using that turned out to be wrong.

6. What is one thing you want gone over in class? Be as specific as you can: "why reset(team) didn't empty the list" is useful, and "strings" is not. Class time gets spent on whatever shows up most often in these answers.

7. Did you use an AI tool this week, and did it build your understanding or replace it? The honest test: could you write PA3's Part C loop again on a blank page, and say what each strip() and split() in it is for? If a video, article, or tutorial made something click, drop the link and one line on why it helped.

8. Paste your PA3 code, all four parts. If one of your string methods or slices surprised you along the way, say which one.