Module 1 · OverviewBy the end of this module you can…

Mental modelThree layers, not one

Mental modelWhat Explorer shows vs. where things really are

Mental modelIt’s all one tree

C:\
├── Program Files\      apps (admin only)
├── Windows\            the OS — hands off
├── Users\
│   └── Jane\            your "home"
│       ├── AppData\     hidden!
│       ├── Desktop\
│       ├── Documents\
│       └── Downloads\
└── actuarial_work\     ← we'll make this
    └── pricing\
        ├── model.py
        └── data\
            └── input.csv

DrivesDrive letters and volumes

H:\  →  \\fileserver01\home\jane
S:\  →  \\corp.local\shared\Actuarial

DrivesOneDrive: helpful for Word, hazardous for code

PathsAnatomy of an absolute path

C:\Users\Jane\projects\model.py

An absolute path starts at the root. It means the same thing no matter where you are.

Downside: it only works on your machine. Your colleague isn’t called Jane.

PathsRelative paths: directions from where you stand

C:\actuarial_work\pricing\
├── data\
│   └── input.csv
├── notebooks\        ← you are here
│   └── explore.ipynb
└── src\
    └── model.py

From notebooks\, the data file is ..\data\input.csv. From the project root it’s just data\input.csv.

Check yourselfResolve these paths

Your CWD is C:\actuarial_work\pricing\src. Where does each point?

SlashesForward /, back \, and escaping

# ❌ Broken: \n and \t are escape sequences
path = "C:\new_data\tables.csv"

# ✅ Option 1: forward slashes (preferred)
path = "C:/new_data/tables.csv"

# ✅ Option 2: a raw string — backslashes taken literally
path = r"C:\new_data\tables.csv"

# ✅ Option 3: escape each backslash
path = "C:\\new_data\\tables.csv"

InterludeMeanwhile, in UNIX-land (Linux, BSD, macOS)

/
├── bin/
├── etc/
├── home/
│   └── jane/          ~
│       └── actuarial_work/
└── mnt/
    └── usb/           a "drive"

InterludeWhy Python wants you to use / — even on Windows

from pathlib import Path

project = Path("C:/actuarial_work/pricing")
data_file = project / "data" / "input.csv"   # "/" joins parts

print(data_file)           # C:\actuarial_work\pricing\data\input.csv
print(data_file.suffix)    # .csv
print(data_file.exists())  # True / False

home = Path.home()         # C:\Users\Jane  (or /home/jane)

ExtensionsThe case of model.py.txt

ExtensionsTurn on extensions & hidden items

Once hidden items are visible, you’ll start seeing programmer furniture:

.git\   .venv\   .gitignore   .python-version   AppData\

Names starting with a dot are a UNIX convention for “hidden”; you’ll create plenty of them in this course.

User profileYour home: C:\Users\<username>

C:\Users\Jane\          %USERPROFILE%, $HOME, ~
├── .local\bin\        uv installs here (Mod 4)
├── AppData\           hidden
│   ├── Local\          caches, per-machine
│   ├── LocalLow\       sandboxed apps
│   └── Roaming\        settings that follow you
├── Desktop\
├── Documents\
├── Downloads\
├── OneDrive\
└── .gitconfig          Git settings (Mod 5)

User profileAppData: where programs keep their stuff

PermissionsFile permission basics

RecapModule 1 checklist

• I can explain disk vs. file system vs. Explorer
• I can read an absolute path and resolve a relative one using . and ..
• I know why "C:\new\table.csv" breaks in Python, and three ways to fix it
• I write / in code (or use pathlib)
• Extensions and hidden items are visible on my machine
• I know where AppData, %USERPROFILE%, and ~ point
• My workspace C:\actuarial_work\ exists, outside OneDrive