Introduction to Python

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Executing Programs and Using the Lab Exercises

creating a new script (.py file) in VS Code

A 'workspace' in VS Code is usually the same as a'project'.

Open a Folder, which will correspond to a new workspace.

• Choose File > Open Folder. A dialog opens to select a folder.
• Choose 'New Folder' to create a folder, or select a folder to open
• A new workspace is shown at the left margin with your folder name as the workspace name

Add a new file.

• Roll over the workspace name on the left, and click the 'new file' icon (the page icon with a 'plus' at the lower left)
• Type the name of your file (with a .py extension) and hit [Enter]. The file appears on the left, and the text area for the file appears in the main window on the right, with the first line highlighted and numbered 1.

• Type the following:

print('hello, world!')
print()

executing a script

VS Code may be able to run your script, or some configuration may be required.

Attempt to run your script.

• At the upper right, click the "run" arrow
• You may see output at the bottom, including the 'hello, world!' text. (Look carefully for it, it may be squeezed in between other output.)
• If you see an error popup indicating that a version of Python must be installed: click 'Select Python Interpreter' at the far lower right, and choose your preferred version of Python. Your version must be numbered 3.8 or greater. Make sure not to select 2.7!
• If you do not see your version of Python listed, please seek assistance from your course manager.

understanding terminal output in VS Code

By default, VS Code passes your code to the Python Interpreter and executes it at the command line.

On my Mac, I see this output:

->  /Users/david/miniconda3/bin/python /Users/david/test_project/test.py
->  (base) DavidBs-MacBook-Pro:test_project david% /Users/david/miniconda3/bin/python /Users/david/test_project/test.py
->  hello, world!

->  (base) DavidBs-MacBook-Pro:test_project david%

• (Note that arrows above are just to indicate each line of output - they will not appear in your display.)
• Your output will look slightly different than mine; for example, on Windows you're likely to see paths starting with C:\ rather than / (forward slash) on Mac/Linux
• The first and second -> lines indicate that I am running Python and feeding my test.py file to it
• Below that is the output of the program
• Lastly, we see the terminal prompt again, indicating the Terminal is ready for the next command

when programs run without error in VSCode terminal

'Without error' means Python did everything you asked.

On my Mac, I see this output:

->  /Users/david/miniconda3/bin/python /Users/david/test_project/test.py
->  (base) DavidBs-MacBook-Pro:test_project david% /Users/david/miniconda3/bin/python /Users/david/test_project/test.py
->  hello, world!

->  (base) DavidBs-MacBook-Pro:test_project david%

• (Again, the arrows above are just to indicate each line of output - they will not appear in your display)
• When you see the terminal prompt repeated, it means that the script has completed executing
• Your path to python will be different than mine, and the % may also be $ or > depending on your system's configuration

when exceptions occur

An 'exception' is raised when Python cannot, or will not, do everything you asked in you program, or doesn't understand what the code means (e.g., because of a SyntaxError).

To demonstrate an exception, I removed one character from my code. Here is the result:

->  /Users/david/miniconda3/bin/python /Users/david/test_project/test.py
->  (base) DavidBs-MacBook-Pro:test_project david% /Users/david/miniconda3/bin/python /Users/david/test_project/test.py
->   File "/Users/david/test_project/test.py", line 2
->     print('hello, world!)
          ^
-> SyntaxError: unterminated string literal (detected at line 2)

(Again, the arrows above are just indicating where each line of output starts - they will not appear in your output.) How should we read our exception?

• First, look at the CamelCase error type: SyntaxError. This tells us the category of error we are witnessing. This error type indicates there is or are character(s) missing or in the wrong place.
• Second, look at the line highlighted by Python: Line 2 (the first print() statement). This is where the error occurred.
• Third, ask yourself why this problem was detected on this line.
• I can't emphasize enough the importance of going straight to the line indicated by Python, and determining why this error occurred there

Throughout this course I will repeatedly call upon you to identify the exception type, pinpoint the error to the line, and seek to understand the error in terms of where Python says it occurred.

the SyntaxError exception

Some element of the code is misplaced or missing.

print('hello, world!)
print()

File "/Users/david/test_project/test.py", line 2
  print('hello, world!)
        ^
SyntaxError: unterminated string literal (detected at line 2)

How do we respond to a SyntaxError? First by understanding that there's something missing or out of place in the syntax (the proper placement of language elements -- brackets, braces, parentheses, quotes, etc.) We look at the syntax on the line, and compare it to similar examples in other code that we've seen. Careful comparison between our code and working code will usually show us what's missing or misplaced. In the example above, the first print() statement is missing a quotation mark. It might be hard to see at first, but eventually you will develop "eyes" for this kind of error. pythonreference.com

writing code: comments and blank lines

Use hash marks to comment individual lines; blank lines are ignored.

 1 # this program adds numbers
 2 var1 = 5
 3 var2 = 2
 4
 5 # add these numbers together
 6 var3 = var1 + var2
 7
 8 # these lines are 'commented out'
 9 # var3 = var3 * 2
10 # var3 = var3 / 20
11
12 print(var3)

• When Python reads this code, it begins by ignoring blank lines
• It also ignores any code to the right of a hash mark (#)
• Text marked with '#' are called 'comments'
• We may want to comment in order to note something in the code (lines 1, 5 and 8), or to disable certain lines temporarily (lines 9 and 10)
• It is a common practice to enable and disable some statements in our code while testing

Opening the Lab Exercises in VS Code

The exercises should be opened as a single folder in VS Code.

python_data
├── 01
├── 02
│   ├── 2.1.py
│   ├── 2.2.py
│   ├── ..etc
│   ├── 2.6_lab.py
│   ├── 2.7_lab.py
│   ├── ..etc
│   └── solutions/

• make sure that your 01, 02, etc. folders are all inside the python_data folder
• in VS Code, close any open project
• choose File > Open Folder... and select your folder
• you should see the PYTHON DATA workspace opened in the left side of your VS Code window
• if you have any difficulty unzipping, finding or opening the python_data folder in VS Code, please contact your course facilitator

Using the Lab Exercises

We will use some exercises for demos in class; you will use them to practice your skills, and prepare for tests.

├── 02
│   ├── 2.1.py       <-- 'journey' exercise
│   ├── 2.2.py
│   ├── ..etc
│   ├── 2.6_lab.py   <-- 'lab' (practice) exercise
│   ├── 2.7_lab.py
│   ├── ..etc
│   └── solutions/

The exercises come in two forms:

• the journey exercises (named 2.1.py, 2.2.py, etc.)
• the lab exercises (named 2.6_lab.py, 2.7_lab.py, etc.)
• we will work through many of the journey exercises as we discover and discuss new features of the language
• you will have the opportunity to practice your skills using the lab exercises
• the exercise_solutions/ folder contains all lab exerise solutions. You are encouraged to try the exercises on your own, and if you get stuck, you may consult the lab exercises.