Python 3

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Introduction to Python

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Executing Programs and Using the Lab Exercises

creating a new script (.py file) in PyCharm

A folder in PyCharm is known as a 'project'.

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

Choose File > Open Folder. A dialog opens to select a folder.
Navigate to your unzipped class folder and select, then click Open.
A new folder 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.

Create a 'hello, world!' script.

Type the following:

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

Take care when reproducing the above script - every character must be in its place. (The print() at end is to clarify the Terminal output.) Next, we'll execute the script.

executing a script

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

Attempt to run your script.

Double-click the .py file. The code from the file appears in a larger window on the right.
In the code window, right-click and choose Run [filename].
You may see output at the bottom, including the 'hello, world!' text.
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 let me know.

when programs run without error

'Without error' means Python did everything you asked.

On my Mac, I see this output:

hello, world!

Process finished with exit code 0

When you see the terminal prompt repeated, it means that the script has completed executing

when exceptions occur

An 'exception' is when Python cannot, or will not, do everything you asked in you program.

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

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

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 stress that you must identify the exception type, pinpoint the error to the line, and seek to understand the error in terms of the exception type, and 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.

writing code: comments and blank lines

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

# this program adds numbers
var1 = 5
var2 = 2

var3 = var1 + var2        # add these numbers together

# these lines modify the value further
# var3 = var3 * 2
# var3 = var3 / 20

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, 4 and 6), or to disable certain lines temporarily (lines 7 and 8)
It is a common practice to enable and disable some statements in our code while testing

Using the Lab Exercises

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

python_data/
├── session_00_test_project/
├── session_01_objects_types/
├──── inclass_exercises/
│       ├── inclass_1.1.py
│       ├── inclass_1.2.py
│       ├── ..etc..
│       ├── inclass_1.6_lab.py
│       ├── inclass_1.7_lab.py
│       ├── ..etc..
├──── notebooks_inclass_warmup/
├── session_02_funcs_condits_methods/
├──── inclass_exercises/
│       ├── inclass_2.1.py
│       ├── inclass_2.2.py
│       ├── ..etc..
├── session_03_strings_lists_files/
├── session_04_containers_lists_sets/
├── ..etc..
└── session_10_classes/

The exercises come in two forms:

the journey exercises (named inclass_2.1.py, inclass_2.2.py, etc.)
the lab exercises (named inclass_2.10_lab.py, 2.10_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 solutions to the exercises are on the website.

[pr]