Lecture 12

Reminders

• Worksheet 5 will be posted this afternoon
• Project 2 description coming soon

Docstrings

A Python function or file can begin with a string literal, a docstring, to document its purpose.

$\texttt{help(function_name)}$ retrieves docstrings.

Any other explanatory text should be a comment.

A function with a docstring:

def f(x):
    "Return the square of `x`"
    return x*x

Getting help for that function:

>>> help(f)
Help on function f in module __main__:

f(x)
    Return the square of `x`
>>>

Explanatory text

Is it the first statement in a file, or the first statement in the body of a function?

• Yes → Use a string literal, so it becomes a docstring
• No → Use a comment (#)

Command line arguments

Taking a break from Python, let's talk about the shell.

When you run a command in the shell, it may accept some strings as arguments, e.g.

PS C:\Users\ddumas> cd Desktop
PS C:\Users\ddumas\Desktop>

Here $\texttt{cd}$ is the command name, and the string $\texttt{Desktop}$ is the first (and only) command line argument (often called an arg).

Command line arguments are separated by spaces.

A Python program can access the command line arguments provided when it was run. For example, if a script is run with the command

python example.py now is the winter of our discontent

Then we can access each string after "python". This is useful so that a program can accept input from the command that runs it, rather than reading it from the keyboard.

To access command line args, we first import the $\texttt{sys}$ module:

import sys

Now we have access to the list $\texttt{sys.argv}$. At index $0$ it contains the name of our script (as given to the interpreter). At index $1$ is the first argument after the script name, etc..

In the previous example, $\texttt{sys.argv}$ would have value:

['example.py', 'now', 'is', 'the', 
 'winter', 'of', 'our', 'discontent']

Example

Let's write a program that repeats a message several times. It should take two command-line arguments:

• Argument 1: Number of times
• Argument 2: Message

Basic version:

"""Repeat a string a given number of times.
The first argument is the number of times.
The second gives the string to repeat.
"""

import sys
n = int(sys.argv[1])
s = sys.argv[2]

for i in range(n):
    print(s)

PS C:\Users\ddumas\Desktop> python repeat0.py 5 hello
hello
hello
hello
hello
hello
PS C:\Users\ddumas\Desktop> python repeat0.py onlyone
Traceback (most recent call last):
  File "repeat0.py", line 7, in <module>
    n = int(sys.argv[1])
ValueError: invalid literal for int() with base 10: 'onlyone'
PS C:\Users\ddumas\Desktop> python repeat0.py
Traceback (most recent call last):
  File "repeat0.py", line 7, in <module>
    n = int(sys.argv[1])
IndexError: list index out of range
PS C:\Users\ddumas\Desktop>

The better version repeat.py checks for too few arguments and handles it gracefully.

"""Repeat a string a given number of times.
The first argument is the number of times.
The second gives the string to repeat.
"""

import sys

if len(sys.argv) < 3:
    print("Usage:",sys.argv[0],"N s")
    print("Prints N copies of string s, one per line.")
else:
    n = int(sys.argv[1])
    s = sys.argv[2]
    for i in range(n):
        print(s)

PS C:\Users\ddumas\Desktop> python repeat.py
Usage: repeat.py N s
Prints N copies of string s, one per line.
PS C:\Users\ddumas\Desktop> python repeat.py 3
Usage: repeat.py N s
Prints N copies of string s, one per line.
PS C:\Users\ddumas\Desktop> python repeat.py 3 goodbye
goodbye
goodbye
goodbye

The handoff of arguments from the shell to the Python script is one of the services of the operating system or OS.

Windows, Linux, Mac OS, Android, iOS are all operating systems.

An OS manages the lowest-level details of a computer's operation.

A key feature of operating systems is that they provide abstraction.

For example: A wireless mouse, a wired mouse, and a touchpad operate very differently. The OS handles these differences so that a program can ask for the current position of the pointer, without concern for the specific hardware.

Glossary

• CPU - Central Processing Unit or processor. The main component of a computer that executes instructions in a computer program.
• Hardware - The physical parts (electronic devices) that make up a computer.
• Software - Collective term for computer programs.

Glossary

• RAM - Random-Access Memory, or just memory. The place where currently-running programs and the data they use are stored. Variables are stored here. Contents of RAM are lost when the computer is powered off or restarted.
• Persistent storage - Hardware devices such as disks, USB flash drives, etc., that can store data that is not lost on restart or power-off.

Some OS services

• Device management: communicate with attached devices (mouse, keyboard, disks, video controller, sound hardware) and provide a standardized interface for them.
• Process management: Control starting, stopping, running of programs as processes. Manage which processes have access to the CPU at a given time.

Some OS services

• Memory management: Processes can only access parts of RAM that the OS allows them to. They can request access to more (or less) RAM.
• File management: Data stored on persistent storage devices is usually arranged into named files, which are in turn arranged into a hierarchy of directories. Storage devices know nothing about these concepts, and store only bytes. The OS provides the file/dir abstractions.

When you click "save"

At a low level (hardware), assuming a wireless mouse:

• Your finger activates a switch in the mouse.
• A processor in the mouse is running a program that frequently checks the switch position. One such check notices it is closed, and calls a function to send notification of the change.
• The mouse begins sending bits of data using a 2.4Ghz radio signal.

• A bluetooth adapter in your computer that is constantly monitoring that radio frequency receives the data and asks for attention from the CPU.
• The CPU pauses the program it was running and switches to a driver in the OS that processes bluetooth data packets.
• The driver analyzes the raw data received over the radio link and adds a new "mouse event" to a list in RAM.

• Later, another part of the OS that notifies programs of user events gets access to the CPU. It begins searching for a process that should receive notification of the new mouse event.
• The editor window is identified as the recipient.
• Later, the editor asks the OS for new events, and gets the mouse event as an answer.

Skipping hundreds of steps until...

• The disk completes the request to write the bytes representing "...print('Hello world')\n" to the specified position.

When you click "save"

At the level of OS-provided functions:

• A loop in the editor is constantly asking the OS if there are new events to handle. Eventually, it receives one—a mouse click.
• The editor determines the click is on "Save".
• A function within the editor to save the current file is called. It determines the filename, and asks the OS to open the file with permission to write.
• The OS gives the editor permission to write to that file.

• The editor takes the contents of the file the user is editing, encodes it into bytes, and asks the OS to write these bytes to the open file.
• The OS reports success, and the editor asks the OS to close the file.
• The editor updates its display to show that the file has no unsaved changes.

Next time

• Reading and writing files in Python
• String formatting