Workshop — session 1

Session 1 workshop — the ledger, by hand

Ungraded. You leave today having typed two short programs, predicted what they print, and been wrong at least once on purpose.

The next session is in two weeks (no class on 21 September). Today we make sure everything works, and we do by hand what the lecture did on the screen.

If you get stuck, raise your hand and say which part you are on.

Part 1 — Open the editor

  1. Open the enrolment link — https://app.eu1.nuvolos.cloud/enroll/class/OqWYtk7RxbA — and sign in with the email you use for Moodle. Do this before Monday if you can. It takes two minutes, and it is the one step that cannot be hurried in a room of 250 people.
  2. Click the app called VSCode. Wait — the first start takes up to a minute. Do not click it again.
  3. You now see the editor. Three areas matter today. Left: the list of files, called the Explorer. Middle: where you type. Bottom: where results appear after you run something.

✅ Part 1 done when you see the editor with the file list on the left.

Part 2 — A folder, a file, a run

A file is text with a name, saved on the computer. A folder is a list of files. A program is a file.

  1. In the file list on the left, put the mouse on the word FILES at the top. Four small icons appear next to it. Click the second one, New Folder (a folder with a plus). Name it dsap — small letters, no spaces. Press Enter.
  2. Click once on dsap so it is selected, then click the first icon, New File. Name it hello.py. Press Enter. The ending .py tells the computer: this file is a Python program.
  3. The file opens in the middle, empty. Type this exactly, including the quotation marks:
print("hello")
  1. Notice: there is no Save to press. This workspace saves every change by itself — a moment after you type, the file on the computer is already updated. (On your own laptop, VS Code does not do this: there you press Cmd+S or Ctrl+S, and an unsaved file shows a white dot on its tab.)
  2. Find the ▷ (play) button at the top right. Click it. A panel opens at the bottom and after a moment shows:
hello

You have written and run a program.

  1. Replace hello with your first name, keeping the quotation marks. Click ▷ again. Your name appears. Change it once more, run once more: what runs is always what is in the file right now.

✅ Part 2 done when hello.py prints your name.

Part 3 — The ledger, by hand

In the lecture you voted on what this program prints. Now type it. Do not copy and paste: typing is the point of this course, and the lecture told you why.

  1. New file in dsap: ledger.py. Type these ten lines (the blank lines too — they do nothing, they are for your eyes):
ticker = "AAPL"
shares = 10
price = 190.50

value = shares * price
print("value at", price, "is", value)

price = 200.00
print("price is now", price)
print("value is still", value)
  1. Before you run it, write on paper what the last line prints.
  2. Run. Compare with your paper:
value at 190.5 is 1905.0
price is now 200.0
value is still 1905.0

If you wrote 2000.0: you read the program as Excel. value = shares * price does not mean “value is always shares times price”. It means “work it out now, and call the answer value”. Then it is done. The price changed afterwards; value did not look back.

  1. Now move one line. Cut price = 200.00 and paste it above value = shares * price. Delete the two print lines that no longer make sense, so the file reads:
ticker = "AAPL"
shares = 10
price = 190.50
price = 200.00

value = shares * price
print("value at", price, "is", value)

Run:

value at 200.0 is 2000.0

Two versions. Both run without any error. Only one of them is your portfolio. Nothing on the computer checks which — that is what the lecture called runs and lies, and it is what you are here to learn to see.

  1. Add two lines at the very end: shares = shares + 5, then print(shares). Predict, run. Read the first one from right to left: take what shares is, add five, put it back under the name shares. It is an instruction, not an equation.

✅ Part 3 done when you have seen 1905.0 and 2000.0 from the same ten lines, and predicted the last one correctly.

Part 4 — Calling things

Look at what you have typed so far. Every line that does something has the same shape:

print("value at", price, "is", value)

A name, then parentheses, and inside them what you hand over, separated by commas. That is a call. print is not a magic word; it is a piece of code somebody else wrote, and the parentheses are how you run it with your own values. Everything in Python that does work looks like this. Libraries, which the lecture called the reason Python won, are shelves of things to call.

There are two kinds. print does something — text appears. Others give something back, and that is where today’s problem hides.

  1. New file in dsap: portfolio.py. Type:
aapl = 10 * 190.50
nesn = 4 * 102.30
voo = 2 * 512.00

total = aapl + nesn + voo
print("total:", total)

share = nesn / total * 100
print("NESN share, in percent:", share)

Run:

total: 3338.2
NESN share, in percent: 12.258103169372717

That last number is correct and unreadable. Python has round for this: hand it a number and how many decimals you want, and it gives back the rounded number.

  1. Leave one blank line, then add two lines at the end:
round(share, 2)
print("rounded:", share)

Predict. Run:

total: 3338.2
NESN share, in percent: 12.258103169372717
rounded: 12.258103169372717

Nothing changed. No error, either. round did its job and handed back 12.26 — and nobody took it. A value that comes back from a call is not kept anywhere unless you give it a name. The line did work and threw the result away.

  1. Fix the line so the answer is kept:
share = round(share, 2)
print("rounded:", share)
rounded: 12.26

Same = as before: call round, take what comes back, put it under the name share. This is the second thing today that ran perfectly and lied. Code written by a chatbot does this one all the time.

✅ Part 4 done when rounded: 12.26 appears, and you can say why the earlier version printed the long number.

Part 5 — Break it, three ways

The lecture said code fails in three ways. You have already met the worst one — it runs and lies — twice. Now the other two, on purpose, because you will see them every week and they are the harmless ones.

  1. It won’t start. In hello.py, delete the second quotation mark. Run. Red text. Do not read it all; read only the last line:
SyntaxError: unterminated string literal (detected at line 1)

Python could not understand line 1, so it did not run anything at all. Put the quotation mark back.

  1. It crashes. In portfolio.py, change the last line to print("rounded:", shar) — one letter missing. Run:
total: 3338.2
NESN share, in percent: 12.258103169372717
Traceback (most recent call last):
  File "/files/dsap/portfolio.py", line 12, in <module>
    print("rounded:", shar)
                      ^^^^
NameError: name 'shar' is not defined. Did you mean: 'share'?

Two lines printed before it stopped: the program ran top to bottom until line 12, then hit a name it had never seen. The last line says what went wrong and even guesses the fix. Fix it.

Every error you ever see will look like this: some lines, then a last line that says what is wrong. Read the last line first. Always.

✅ Part 5 done when you have seen both messages and both files run again.

Part 6 — Where your file is

The lecture said a file is bytes plus a name, and the name lives in a tree. Look at yours.

  1. In the file list, put the mouse on hello.py and hold it there. A small label appears: /files/dsap/hello.py. That is the absolute path: start at the root /, go into files, then dsap, and there is the file. Every file in this workspace has one, and it means the same thing from anywhere.
  2. Click once on hello.py, press F2, and rename it to hello.txt. Press Enter. Look: the colours in the text are gone, and the ▷ button is gone. Nothing inside the file changed — every byte is the same. The ending only tells the editor what to expect. This is the lecture’s “Windows hides extensions” trap, seen from the other side.
  3. F2 again, back to hello.py. Colours and ▷ return.

✅ Part 6 done when you can say the full path of hello.py, and hello.py runs again.

Part 7 — The three things hiding in plain sight

The lecture showed them on the screen. Now your computer says them.

New file in dsap: float.py. Type these three lines and run:

print(0.1 + 0.2)
print(0.1 + 0.2 == 0.3)
print(round(0.5), round(1.5), round(2.5), round(3.5))
0.30000000000000004
False
0 2 2 4

Line 1: 0.1 cannot be written exactly in binary, so the computer stores a very close neighbour and adds those. Line 2: which is why you never compare money with ==. Line 3: round breaks ties toward the even number — the banking standard. Not a bug, not a beginner problem; this happens to everyone, forever.

One more line, then run again:

print(0.1 + 0.2 + 0.3, 0.3 + 0.2 + 0.1)
0.6000000000000001 0.6

Same numbers, different order, different answer. Session 2 shows what to do about it.

✅ Part 7 done when all four lines have printed and nothing surprised you the second time.

Before 28 September

Come back once, on any day, for ten minutes: open Nuvolos, open VSCode, open portfolio.py, change a number, run. If it works, you are ready for session 2. If it does not, write down what the screen said and bring it.

If you finished early