Python for Actuarial Students · Course 0W

Module 3

Environment Variables, the System PATH & Software Installation

How Windows finds programs, why “not recognized” happens, and how to install software the programmer’s way with winget.

Module 3 · OverviewBy the end of this module you can…

Describe what happens between typing a command and it running
Explain environment variables and read them with $env:
Read, edit, and repair your PATH — GUI and PowerShell
Tell user variables from system variables
Diagnose “is not recognized” errors methodically
Recognize executable types and set a sensible execution policy
Install software with installers and with winget

50 min

Concepts & demos

35 min

Build your own global command

Part 1

How Windows Finds Programs

You type python. What actually happens?

The lookupWhat happens when you press `Enter`

1. Built-in?Is python a PowerShell alias, function, or cmdlet?

no →

2. Search PATHLook in each folder listed in PATH, in order

→

3. Try extensionsIn each folder try python.exe, .cmd, .bat, .ps1…

→

4. First hit winsRun it. Stop searching.

Nothing found in any folder?
python: The term 'python' is not recognized as a name of a cmdlet, function, script file, or executable program.

PowerShell does not look in the current folder. To run build.ps1 sitting right here, type .\build.ps1.

ConceptEnvironment variables

Named text values that every program inherits when it starts — shared settings for the whole session.

PS> $env:USERNAME
Jane
PS> $env:USERPROFILE
C:\Users\Jane
PS> ls env:          # list them all

Variable	Typical value
`USERPROFILE`	`C:\Users\Jane`
`APPDATA`	`…\AppData\Roaming`
`LOCALAPPDATA`	`…\AppData\Local`
`TEMP`	`…\AppData\Local\Temp`
`COMPUTERNAME`	`LAPTOP-7F3K`
`PATH`	the list of program folders

Syntax differs by shell: $env:TEMP in PowerShell, %TEMP% in cmd & Explorer, os.environ["TEMP"] in Python.

Concept`PATH`: the phone book for programs

One long string of folders separated by ;

Windows reads it top to bottom and runs the first match.

Split it into lines to make it readable:
$env:PATH -split ';'

PS> $env:PATH -split ';'
C:\Program Files\PowerShell\7
C:\WINDOWS\system32
C:\WINDOWS
C:\WINDOWS\System32\WindowsPowerShell\v1.0\
C:\Program Files\Git\cmd
C:\Users\Jane\.local\bin
C:\Users\Jane\AppData\Local\Microsoft\WindowsApps
C:\Users\Jane\AppData\Local\Programs\Microsoft VS Code\bin

ConceptOrder matters: the case of the fake `python`

Fresh Windows installs contain a stub python.exe in WindowsApps that just opens the Microsoft Store.

Ask PowerShell which one wins:

PS> Get-Command python -All | Select-Object Source

Source
------
C:\Users\Jane\AppData\Local\Microsoft\WindowsApps\python.exe

where.exe python does the same job and works in cmd too.

Get-Command answers the question “which program will actually run?” — the first thing to check whenever a tool behaves strangely.

Turn the stub off: Settings → Apps → Advanced app settings → App execution aliases → switch off python.exe and python3.exe. In Module 4, uv will give us real Pythons.

ScopeUser variables vs. system variables

User variables

Apply only to you
Edit without admin rights
Where per-user tools (uv, VS Code) add themselves
This is where we work.

System variables

Apply to every user on the PC
Require administrator rights
Where machine-wide installs (Git, Windows) live
Look, don’t touch.

System PATH

User PATH

$env:PATHwhat each new terminal sees

LifetimeChanges only reach new windows

Each program copies the environment when it starts. After you change a variable permanently:

Terminals already open still have the old copy
VS Code (and its terminals) still has the old copy

Changed PATH? Close and reopen the terminal — every time.

Set with…	Lasts
`$env:X = "…"`	This terminal window only — gone when closed
Environment Variables dialog	Permanent, all future programs
`[Environment]::SetEnvironmentVariable(…, "User")`	Permanent, all future programs

EditingThe GUI way

Press Win, type “environment”, choose Edit environment variables for your account (no admin needed)
Top half = User variables. Select Path → Edit…
New → paste a folder path → Move Up/Down to set priority
OK, OK — then open a new terminal

Shortcuts that open the same dialog:

rundll32 sysdm.cpl,EditEnvironmentVariables

sysdm.cpl → Advanced tab → Environment Variables… (this route shows System variables too, which need admin to change)

Never paste over the whole Path value. Add or remove one entry at a time.

EditingThe PowerShell way

# Read
$env:PATH -split ';'
[Environment]::GetEnvironmentVariable("Path", "User") -split ';'   # just the User part

# Temporary — this window only (great for experiments)
$env:PATH = "C:\actuarial_work\bin;" + $env:PATH
$env:MY_DATA_DIR = "C:\actuarial_work\data"

# Permanent — append a folder to the USER Path
$bin      = "C:\actuarial_work\bin"
$userPath = [Environment]::GetEnvironmentVariable("Path", "User")
[Environment]::SetEnvironmentVariable("Path", "$userPath;$bin", "User")

The permanent version writes to the registry, exactly like the GUI. Run it once; running it twice adds the folder twice (harmless, but messy).

Troubleshooting“… is not recognized” — a diagnostic checklist

Typo? Try Tab-completing the command name.
Installed? Find the actual .exe on disk (installer docs, winget list).
Which folder? Note the folder that contains it, e.g. C:\Users\Jane\.local\bin.
On PATH? $env:PATH -split ';' — is that folder listed?
Stale window? Open a new terminal and try again.
Still no? Add the folder to your User Path and repeat step 5.

PS> uv --version
uv: The term 'uv' is not recognized…

PS> Test-Path ~\.local\bin\uv.exe
True
PS> $env:PATH -split ';' |
>>   Select-String local
(nothing)

Diagnosis: installed, but its folder isn’t on PATH in this window.

ExecutablesWhat counts as a “program”?

Extension	What it is	Run by
`.exe`	Compiled program — `python.exe`, `git.exe`, `uv.exe`	Windows directly
`.cmd` / `.bat`	Batch script — old-style command list	`cmd.exe`
`.ps1`	PowerShell script	PowerShell — if execution policy allows
`.py`	Python script	Python: `python script.py` / `uv run script.py`

The variable PATHEXT lists which extensions Windows tries automatically: $env:PATHEXT → .COM;.EXE;.BAT;.CMD;…

ExecutablesExecution policy: why your `.ps1` won’t run

PS> .\mkproj.ps1
.\mkproj.ps1 cannot be loaded because running
scripts is disabled on this system.

Windows client PCs default to Restricted. The recommended setting for developers:

Set-ExecutionPolicy -Scope CurrentUser RemoteSigned
Get-ExecutionPolicy -List      # verify

Policy	Meaning
Restricted	No scripts at all
RemoteSigned	Your own scripts run; downloaded scripts must be signed
Unrestricted	Everything runs (warns on downloads)
Bypass	No checks — used for one-off commands

-Scope CurrentUser = no admin needed, affects only you. On managed work PCs, IT policy may override this.

Part 2

Installing Software

Point-and-click installers vs. the winget package manager.

InstallingTwo ways to install

Downloadable installer (`.exe` / `.msi`)

Find the official site, pick the right download
Click through a wizard — some screens matter (e.g. “Add to PATH”)
Updates: remember to come back
Easy to grab a fake from a search ad

Package manager (`winget`)

One command; built into Windows 10/11
Pulls from a curated catalog of official installers
winget upgrade --all updates everything
Scriptable — a whole setup in one file

Either way, most installers edit your PATH. That’s why this lives in the PATH module — and why you open a new terminal after every install.

wingetEveryday `winget`

winget search pandoc                  # find packages
winget show --id JohnMacFarlane.Pandoc   # details
winget install --id JohnMacFarlane.Pandoc -e
winget list                           # what's installed
winget upgrade                        # what's out of date
winget upgrade --all                  # update everything
winget uninstall --id JohnMacFarlane.Pandoc

IDs we’ll use in this course

Microsoft.PowerShell

astral-sh.uv

Git.Git

JohnMacFarlane.Pandoc

Microsoft.VisualStudioCode

--id … -e means “this exact ID”, avoiding look-alike packages. The first run may ask you to accept the source agreements — read, then type Y.

Hands-on Lab · 35 minBuild your own global command

Goal: type mkproj pricing_2026 from any folder and get a ready-made project skeleton.

Create a personal script folder: C:\actuarial_work\bin
Allow your own scripts to run (execution policy)
Write the tiny utility mkproj.ps1
Add the folder to your User PATH
Open a new terminal and call it from anywhere

Lab · Steps 1–3Folder, policy, script

mkdir C:\actuarial_work\bin
Set-ExecutionPolicy -Scope CurrentUser RemoteSigned
notepad C:\actuarial_work\bin\mkproj.ps1        # say "Yes" to create the file

Type this into Notepad and save:

# mkproj.ps1 — create a standard actuarial project skeleton
param([Parameter(Mandatory)] [string] $Name)

$folders = "data", "notebooks", "src", "output", "tests"
foreach ($f in $folders) {
    New-Item -ItemType Directory -Force -Path (Join-Path $Name $f) | Out-Null
}
Write-Host "Created $Name with: $($folders -join ', ')" -ForegroundColor Green

Check with ls C:\actuarial_work\bin that it’s mkproj.ps1 — not mkproj.ps1.txt (Module 1 flashback!).

Lab · Steps 4–5Put it on the PATH and test

4 · Add to User PATH (pick one)

GUI: Edit environment variables for your account → Path → New → C:\actuarial_work\bin

PowerShell:

$bin = "C:\actuarial_work\bin"
$u = [Environment]::GetEnvironmentVariable("Path","User")
[Environment]::SetEnvironmentVariable("Path","$u;$bin","User")

5 · New terminal, then:

PS C:\Users\Jane> cd C:\actuarial_work
PS C:\actuarial_work> mkproj pricing_2026
Created pricing_2026 with: data, notebooks,
src, output, tests
PS C:\actuarial_work> Get-Command mkproj

CommandType  Name        Source
-----------  ----        ------
ExternalScript mkproj.ps1 C:\actuarial_work\bin\mkproj.ps1

Bonus: install something with winget (e.g. winget install --id JohnMacFarlane.Pandoc -e — we need it in Module 6), then find which PATH entry it added.

RecapModule 3 checklist

I can explain how a command name becomes a running program
I can list environment variables and print one with $env:NAME
I can read my PATH one entry per line
I know user vs. system variables, and I edit only user ones
After changing PATH or installing software, I open a new terminal
I can diagnose “not recognized” with Get-Command and Test-Path
My execution policy is RemoteSigned for CurrentUser
I can install, list, and upgrade software with winget
mkproj works from any folder

Up next

Module 4

Next-Gen Python Tooling with uv

Python itself, virtual environments, and reproducible packages — installed per-user, no admin, no PATH surgery.

← Course home Next: Module 4 →

Environment Variables, the System PATH & Software Installation

Module 3 · OverviewBy the end of this module you can…

How Windows Finds Programs

The lookupWhat happens when you press Enter

ConceptEnvironment variables

ConceptPATH: the phone book for programs

ConceptOrder matters: the case of the fake python

ScopeUser variables vs. system variables

User variables

System variables

LifetimeChanges only reach new windows

EditingThe GUI way

EditingThe PowerShell way

Troubleshooting“… is not recognized” — a diagnostic checklist

ExecutablesWhat counts as a “program”?

ExecutablesExecution policy: why your .ps1 won’t run

Installing Software

InstallingTwo ways to install

Downloadable installer (.exe / .msi)

Package manager (winget)

wingetEveryday winget

IDs we’ll use in this course

Hands-on Lab · 35 minBuild your own global command

Lab · Steps 1–3Folder, policy, script

Lab · Steps 4–5Put it on the PATH and test

4 · Add to User PATH (pick one)

5 · New terminal, then:

RecapModule 3 checklist

Next-Gen Python Tooling with uv

The lookupWhat happens when you press `Enter`

Concept`PATH`: the phone book for programs

ConceptOrder matters: the case of the fake `python`

ExecutablesExecution policy: why your `.ps1` won’t run

Downloadable installer (`.exe` / `.msi`)

Package manager (`winget`)

wingetEveryday `winget`