What Python installs

The pieces

One download, several tools. Here is what lands on your machine.

The interpreter

The program that reads your Python and carries it out. Everything else is built around it.

Open a terminal. On Windows that is Command Prompt or PowerShell; on macOS it is Terminal. Then start Python:

# Windows
py
# or
python

# macOS
python3

You now see the >>> prompt. This is the interactive shell: type an expression, press Enter, see the result. Type exit() or press Ctrl+Z then Enter (Windows) or Ctrl+D (macOS) to leave.

To run a program you saved in a file, pass the file name instead:

# Windows
python hello.py

# macOS
python3 hello.py

Two checks that are always useful:

python --version          # which version am I running
Python 3.12.4

# inside the >>> shell: where is this interpreter, and
# which folders does it search when I import something
>>> import sys
>>> sys.executable
'C:\\Users\\you\\AppData\\Local\\Programs\\Python\\Python312\\python.exe'
>>> sys.path
['', 'C:\\...\\python312.zip', 'C:\\...\\Lib', 'C:\\...\\site-packages']

From the terminal you can also ask the operating system: where python on Windows, which python3 on macOS.

The standard library and modules

A module is a file of ready made code. The standard library is the big set of modules that comes with Python, so you can use them without installing anything.

You bring a module in with import, then reach into it with a dot:

>>> import math
>>> math.sqrt(2)
1.4142135623730951
>>> math.pi
3.141592653589793

>>> from random import randint
>>> randint(1, 6)
4

>>> import statistics
>>> statistics.mean([3, 5, 7, 9])
6

To see what is inside a module, or read its documentation:

>>> import math
>>> dir(math)                # every name the module provides
['acos', 'acosh', ..., 'sqrt', 'tan', 'tau', 'trunc']
>>> help(math)               # the full manual page for the module
>>> help(math.sqrt)          # just one function

>>> import random
>>> random.__file__          # where the module file lives on disk
'C:\\...\\Python312\\Lib\\random.py'

A few modules such as sys and math are compiled into the interpreter itself and have no .py file, so sys.__file__ does not exist. You can see that group with sys.builtin_module_names.

Modules you will meet early

IDLE

IDLE stands for Integrated Development and Learning Environment. It is the small editor that ships with Python, and it is what we use in CIS 2020.

It is itself a Python program, written with the tkinter module, so it looks the same on Windows and macOS.

Opening it:

• Windows: Start menu, search for IDLE. Or from a terminal: py -m idlelib
• macOS: Applications folder, Python 3.x, then IDLE. Or from a terminal: python3 -m idlelib

The two windows:

• The Shell opens first. It is the same >>> prompt as the interpreter, good for trying one line at a time.
• File, New File opens the Editor. Write your program, save it with a .py name, then press F5 to run it. The Shell shows the output and restarts fresh each run.

Useful settings live under Options, Configure IDLE: turn on line numbers, set the font size, and change indentation width.

Reserved words (keywords)

Keywords are the words that are part of Python's grammar, such as if, for, and def. You cannot use them as variable names. Modern Python 3 has 35 of them, though the exact count has shifted across versions, so check rather than memorize.

The quickest look, from the >>> prompt:

>>> help("keywords")
Here is a list of the Python keywords.  Enter any keyword to get more help.

False     await     else      import    pass
None      break     except    in        raise
True      class     finally   is        return
and       continue  for       lambda    try
as        def       from      nonlocal  while
assert    del       global    not       with
async     elif      if        or        yield

The keyword module gives you the same information as data you can test against:

>>> import keyword
>>> keyword.kwlist
['False', 'None', 'True', 'and', 'as', 'assert', 'async', 'await',
 'break', 'class', 'continue', 'def', 'del', 'elif', 'else', 'except',
 'finally', 'for', 'from', 'global', 'if', 'import', 'in', 'is', 'lambda',
 'nonlocal', 'not', 'or', 'pass', 'raise', 'return', 'try', 'while',
 'with', 'yield']
>>> len(keyword.kwlist)
35
>>> keyword.iskeyword("for")
True
>>> keyword.iskeyword("range")
False
>>> keyword.softkwlist       # words reserved only in certain places
['_', 'case', 'match', 'type']

Built in functions

These are the functions you can call without importing anything: print, len, range, input, int, float, str, type, abs, round, sum, min, max, sorted, and more.

They live in a module called builtins that Python imports for you automatically every time it starts. To see the whole set:

>>> dir(__builtins__)         # every name that is always available
['ArithmeticError', ..., 'abs', 'aiter', 'all', 'any', 'ascii', 'bin',
 'bool', 'bytearray', ..., 'print', 'range', 'repr', 'reversed', 'round',
 'set', 'setattr', 'slice', 'sorted', 'str', 'sum', 'tuple', 'type',
 'vars', 'zip']

>>> help("builtins")          # the manual: functions, types, and exceptions

That list also contains the built in types (int, str, list) and every exception name. The clean list of just the functions, about seventy of them, is in the documentation under Built in Functions at docs.python.org.

pip

pip is the tool that installs packages other people have written. The standard library is large, but pip reaches the roughly half a million extra packages on PyPI, the Python Package Index, such as requests, pandas, and matplotlib.

The name is a short joke on "pip installs packages". It has come bundled with Python since version 3.4, so if you have Python you have pip.

Check that it is there

# Windows
py -m pip --version

# macOS
python3 -m pip --version

The safest way to call pip is py -m pip on Windows or python3 -m pip on macOS. Writing it that way runs pip with the exact interpreter you named, so packages land where you expect. A bare pip command can point at a different Python than the one you are using.

The commands you will use

Put py -m (Windows) or python3 -m (macOS) in front of each of these:

pip install requests               # install the newest version
pip install pandas==2.2.2          # install one exact version
pip install --upgrade requests     # move a package up to the newest
pip install --upgrade pip          # update pip itself
pip list                           # everything installed right now
pip show requests                  # version, location, dependencies
pip uninstall requests             # remove a package
pip install -r requirements.txt    # install a project's whole list

Windows and macOS, side by side

# Windows
py -m venv venv
venv\Scripts\activate

# macOS
python3 -m venv venv
source venv/bin/activate

Try these now

Start the interpreter and paste these one at a time.

import sys;     print(sys.version)          # your exact Python build
import keyword; print(keyword.kwlist)       # the reserved words
print(dir(__builtins__))                    # the always available names
import this                                 # the Zen of Python, an easter egg module
help()                                      # interactive help; type quit to leave