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The Way Of The Program

Learning objectives

  • Practice using the UPPMAX documentation
  • Practice using the Python book How to Think Like a Computer Scientist: Learning with Python 3
  • Get a first impression of what the Python programming language is
  • Know the basics of what the Python interpreter is
  • Get a first idea of what a program is
  • Hear about the three types of errors
  • Build up experience with the Python interpreter
  • Use a text editor on an UPPMAX HPC cluster
  • Create a Python script
  • Run a Python script
For teachers

Teaching goals are:

  • Learners have a general idea about what the Python programming language is
  • Learners have used the word 'Python interpreter'
  • Learners have a general idea about what a program is
  • Learners have used the terms syntax error, runtime error and semantic error
  • Learners have run code in the Python interpreter

Other goals are:

  • Verify that learners indeed have learned how to create a file
  • Verify that learners indeed have learned how to use a text editor

Lesson plan:

  • 5 mins: prior knowledge
  • 5 mins: presentation
  • 15 mins: challenge
  • 5 mins: feedback

Prior questions:

  • What types of programming errors exist?
  • Can you name of types of programming bugs?
  • What is a script?
  • Can you name a text editor?

Why?

At the start of learning Python, there are many new terms. Using a book to learn these is a good start.

What is Python? What is a programming language? What type of errors will I write when programming? How do I write code in Python?

These fundamental question are discussed and practiced in this session.

flowchart TD
    python[[Python]]

    %% Give a white background to all nodes, instead of a transparent one
    classDef node fill:#fff,color:#000,stroke:#000

    subgraph sub_programming_language[Python as a language]
      interpreted_language[Interpreted language]
      programming_language[Programming language]
      scripting_language[Scripting language]
      interpreter[Interpreter]
      scripts[Scripts]
      text_files[Text files]
    end
    style sub_programming_language fill:#ccf,color:#000,stroke:#fcc


    subgraph sub_uppmax[UPPMAX]
      uppmax[UPPMAX]
      uppmax_modules[UPPMAX modules]
      uppmax_clusters[UPPMAX clusters]
      text_editors[Text editors]
      %%x_forwarding[X-forwarding]
      %%remote_desktop[Remote desktop]
    end
    style sub_uppmax fill:#fcc,color:#000,stroke:#fcc

    python --> |is a| programming_language
    python --> |is a| interpreted_language
    python --> |is a| scripting_language

    programming_language --> |uses| text_files
    interpreted_language --> |has a| interpreter
    scripting_language --> |is a|interpreted_language
    scripting_language --> |runs| scripts
    scripts --> |are| text_files
    %% scripts --> |can use| command_line_arguments

    uppmax --> |has| uppmax_clusters
    uppmax_clusters --> |have| uppmax_modules
    uppmax_clusters --> |have| text_editors
    %%uppmax_clusters --> |allow| x_forwarding
    %%uppmax_clusters --> |has| remote_desktop
    %% uppmax_modules --> |allow the use of| python_packages
    uppmax_modules --> |allow the use of| interpreter

    text_editors --> |work on|text_files
    %%x_forwarding --> |allows|graphics
    %%remote_desktop --> |allows|graphics


    %% variables ~~~ uppmax

Exercises

Enjoy a video?

You can find a video with solutions to these exercises here

See the exercise procedure here.

Exercise 1: Python and the Python interpreter

Learning objectives

  • Get a first impression of what the Python programming language is
  • Know the basics of what the Python interpreter is
  • Get a first idea of what a program is
  • Hear about the three types of errors
  • Build up experience with the Python interpreter

Read the following sections of How to Think Like a Computer Scientist: Learning with Python 3:

  • 1.1 The Python programming language
  • 1.2. What is a program?
  • 1.4. Syntax errors
  • 1.5. Runtime errors
  • 1.6. Semantic errors
  • 1.9. The first program

Then in section 1.12, do exercises 1 to and including 5. For your convenience, the questions are copy-pasted below:

1.12.1

Write an English sentence with understandable semantics but incorrect syntax. Write another English sentence which has correct syntax but has semantic errors.

Answer

From the Wikipedia link: 'A syntax error is an error in the syntax of a sequence of characters that is intended to be written in a particular programming language'.

An English sentence with understandable logic but incorrect syntax:

One plus one is twoooooooo

A semantic error is also called a logic error, hence a sentence with correct syntax and semantic errors is:

From here, where the sentence is attributed to Noam Chomsky:

Colorless green ideas sleep furiously

From here:

My mother's white cat is black.

1.12.2

Using the Python interpreter, type 1 + 2 and then hit return. Python evaluates this expression, displays the result, and then shows another prompt. * is the multiplication operator, and ** is the exponentiation operator. Experiment by entering different expressions and recording what is displayed by the Python interpreter.

Answer

This is more about experimenting with the operators +, * and **. Here are some things to let the Python interpreter evaluate:

1+2
1 + 2
1*2
2 * 3
2**3

1.12.3

Type 1 2 and then hit return. Python tries to evaluate the expression, but it can't because the expression is not syntactically legal. Instead, it shows the error message:

File "<interactive input>", line 1
1 2
  ^
SyntaxError: invalid syntax

In many cases, Python indicates where the syntax error occurred, but it is not always right, and it doesn't give you much information about what is wrong.

So, for the most part, the burden is on you to learn the syntax rules.

In this case, Python is complaining because there is no operator between the numbers.

See if you can find a few more examples of things that will produce error messages when you enter them at the Python prompt. Write down what you enter at the prompt and the last line of the error message that Python reports back to you.

Answer

This question is more about experimenting, armed with limited knowledge on +, *, ** and print.

Here are some of my favorites:

>>>  1
  File "<stdin>", line 1
    1
IndentationError: unexpected indent

>>> print 1
  File "<stdin>", line 1
    print 1
    ^^^^^^^
SyntaxError: Missing parentheses in call to 'print'. Did you mean print(...)?

>>> print 1 + 1
  File "<stdin>", line 1
    print 1 + 1
    ^^^^^^^^^^^
SyntaxError: Missing parentheses in call to 'print'. Did you mean print(...)?

>>> print(1) + print(1)
1
1
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for +: 'NoneType' and 'NoneType'

>>> 1++
  File "<stdin>", line 1
    1++
       ^
SyntaxError: invalid syntax

1.12.4

Type print("hello"). Python executes this, which has the effect of printing the letters h-e-l-l-o. Notice that the quotation marks that you used to enclose the string are not part of the output. Now type "hello" and describe your result. Make notes of when you see the quotation marks and when you don’t.

Answer

Here are the four combinations of using print yes/no and using single/double quotes:

>>> print("Hello")
Hello
>>> print('Hello')
Hello
>>> "Hello"
'Hello'
>>> 'Hello'
'Hello'

A correct conclusion is:

  • When using print, any type of quotes is removed
  • When not using print, the word is printed in single quotes

1.12.5

Type cheese without the quotation marks. The output will look something like this:

Traceback (most recent call last):
  File "<interactive input>", line 1, in ?
NameError: name 'cheese' is not defined

This is a run-time error; specifically, it is a NameError, and even more specifically, it is an error because the name cheese is not defined. If you don't know what that means yet, you will soon.

Answer

This is more of an explanation. Indeed:

>>> cheese
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'cheese' is not defined

Exercise 2: Running a Python script

Learning objectives

  • create a Python script
  • run a Python script
  • use a text editor on an UPPMAX HPC cluster

1.12.6.1

Type 6 + 4 * 9 at the Python prompt and hit enter. Record what happens.

Answer 
>>> 6 + 4 * 9
42

1.12.6.2

Now create a Python script with the following contents:

6 + 4 * 9

What happens when you run this script?

Tip: search the UPPMAX documentation for a text editor. Use the simplest one.

Answer

Text editors are documented at https://docs.uppmax.uu.se/software/text_editors/.

The simplest text editor is called nano and is documented at https://docs.uppmax.uu.se/software/nano/.

Creating a Python script my_script.py using nano:

nano my_script.py

The content of the file should be:

6 + 4 * 9

Running the Python script my_script.py:

python my_script.py

1.12.6.3

Now change the script contents to:

print(6 + 4 * 9)

and run it again.

What happened this time?

Answer

Modifying the Python script again:

nano my_script.py

The content of the file should be:

print(6 + 4 * 9)

Running the Python script my_script.py:

python3 my_script.py

Gives:

42

1.12.6.4

Whenever an expression is typed at the Python prompt, it is evaluated and the result is automatically shown on the line below (Like on your calculator, if you type this expression you'll get the result 42).

A script is different, however. Evaluations of expressions are not automatically displayed, so it is necessary to use the print function to make the answer show up.

It is hardly ever necessary to use the print function in immediate mode at the command prompt.

Answer

There is no question there :-)