Using a code editor to write Python
The page will get you started writing a Python script by editing it in a code editor, saving it in a file, then running the script via the command line. An alternate method is to do the code development using an Integrated Development Environment (IDE). There are instructions for getting started with an beginner’s IDE called Thonny on this page.
*Note: these instructions assume that you have already installed a distribution of Python on your computer. If you haven’t done that yet, see the Instructions for installing Python first.
Chosing a code editor
There are many excellent free code editors available that can help you more easily create Python scripts. Some of them are described at the Guru99 website. Two will be mentioned here: Atom and Visual Studio Code (not part of Microsoft’s Visual Studio IDE). They are not necessarily better than all the others, but we have experience with them and can recommend them. Both editors are available for Mac, Windows, and Linux, so if you get used to one, you can continue to use it if you change platforms at some time in the future.
Syntax highlighting
All good code editors provide syntax highlighting based on the particular programming language of the file you are editing. Editors generally figure out the language you are using from the file extension of the file. So if you open a file that has the extension .py (the usual file extension for Python scripts), the editor will provide highlighting specifically for Python. If you open a new file and start typing, the editor will not know what kind of highlighting to use until the first time you save the file. So usually you will want to save the file (with the .py extension) as soon as you start writing the code.
Here’s what syntax highlighting looks like in Atom for a Python script:
Here’s what the syntax highlighting looks like for the same script in Visual Studio Code:
Indentation!
An important feature of the Python language is that indentation is the primary means for designating structure in the program. That’s in contrast to other languages where characters such as curly brackets ({ and }) are used to demarcate blocks of code.
The standard for Python is to set off code blocks by indenting them four space characters. However, the Python interpreter will allow any kind of indentation, as long as it is consistent within a block of code. Unfortunately, it is possible to mix indentations using tab characters with indentations using multiple spaces in the same code block. The two will look similar visually, but mixing the spaces and tabs will cause an error when the code is run.
Fortunately, both Atom and Visual Studio Code are pretty smart about dealing with indentation. When a file is opened, they detect whether the indentations were made consistently with tab characters or space characters. When the tab key is pressed they match the kind of character that was previously used in that file, inserting a tab character if tabs were previously used and inserting spaces if spaces were previously used. When auto-indenting after you press the return/enter key, they also will match the characters used in the previous line. In a new Python file, pressing the tab key will automatically default to the preferred four spaces per indent.
If you have previously used a “dumb” editor to edit a script with inconsistent use of tabs and spaces, Atom and Code can’t help you with that. So it’s best to use a “smart” editor from the start.
Pros and cons of the editors
Here are a few ad hoc observations that might help you chose between Atom and V.S. Code.
Atom has a nice build-in Markdown previewer. You can also install a package that enables GitHub integration within the editor. (Atom was created by the GitHub people.) However, Atom also has an annoying navigator pane that sometimes appears at inconvenient times (but that can easily be closed by Ctrl \ in Windows or option \ in Mac).
When editing Python, V.S. Code makes faint guidelines to make the indentation levels more apparent (see the screenshot above). This can be helpful in long scripts. It also shows a miniature view of the whole script at the right side of the screen, and you can click and drag on the miniscript view to move rapidly through the whole script. V.S. Code has debugging features that let you step through your Python code and see what’s going on as the program runs.
Both editors have autocomplete features, which can be awesome or annoying depending on what you like.
If you want, you can install both editors and compare them. There is no problem with editing code with one of them and at any time switching to editing that code with the other.
Installing the code editor
To install Visual Studio Code, go to https://code.visualstudio.com/. You browser should detect your operating systme and suggest the correct download for it.
To install Atom, go to https://atom.io/. You browser should detect your operating systme and suggest the correct download for it.
Workflow for editing and testing Python code
1. Start up your code editor and open a new window.
2. Type or paste this code into the window:
number = 5
divisor = 6
print(number/divisor)
It should look something like this:
Notice that there’s no syntax highlighting because the editor doesn’t know what kind of file it is yet.
3. We need to save this script as a file in a place where we can find it using the command line console. The easiest way to accomplish this is to save the file in your user home directory.
3a. Windows: In the Save As dialog, navigate to your hard drive (usually c:), then the Users directory, then the directory named after your username
When you have navigated to your home directory, select Python as the file type (if necessary) and name the file divide_numbers.py.
Note: in Python, it is best not to use hyphens to separate words in file names. It’s better to use underscores for that purpose. Do not leave spaces in the file names either.
3b. Mac: In the Save As dialog, click on your home folder in the Favorites list (the home folder is indicated by a little house icon). If the dialog box doesn’t show the more extensive file navigation interface, click on the button to the right of the directory name (circled here in red) to make it show up.
Select Python as the file type (if necessary) and name the file divide_numbers.py.
Note: in Python, it is best not to use hyphens to separate words in file names. It’s better to use underscores for that purpose. Do not leave spaces in the file names, either.
4. You should now see Python syntax highlighting (colored text) in the code editor:
5. Open the appropriate console for your operating system (Command Prompt for Windows or Terminal for Mac). If you don’t know how to do that, see these instructions for mac or these instructions for Windows). However, do not launch the Python shell. (If you do launch it accidentally and see the >>> Python prompt, enter quit() to stop the shell.)
6a. Windows: Enter the command dir to do a listing of the home directory. You should see the name of the file you just saved.
6b. Mac: Enter the command ls to do a listing of the home directory. You should see the name of the file you just saved.
7. If you don’t see your file, you may not have actually saved the file in your home directory. You can either figure out how to save the file in your home directory, or save the file to some other known directory and use the cd command line command to change to that directory. Get help from someone if necessary.
8. In the console, run the script. In Windows, enter the command
python divide_numbers.py
On a Mac, enter the command
python3 divide_numbers.py
Note: in all subsequent examples, when running a script you will always need to put the “3” after the “python” if you are using a Mac, even if the example doesn’t show it.
The console should print the number 0.8333333333333334, then show the command prompt again, like this in Windows:
or this on a Mac:
Notice that the console does not output any information other than what the script said to print in line 3. There is no indiction that any other lines of the code had executed. Also note that unlike in the Python shell, you cannot execute individual Python commands, or examine the values of variables by printing them. When developing code in this way, you can only run the entire script as it exists in a saved file.
9. To modify the script, return to the code editor, make changes to the code and save the file again. Note: simply changing lines in the code editor will not cause changes when the script is run unless the file is saved! In this example, change line 2 to
divisor = 2
10. To see the effect of the changes, return to the console and run the file again as you did before. Note: you can bring up the previous command in the console by pressing the up arrow key once, then pressing Enter/Return.
Here’s what should happen:
Again, only the results of the print command are displayed in the console output.
This process of changing and re-running a script is one method of developing and testing code. Usually, during development of a long script you will want to print many intermediate results so that it’s possible to have a better understanding of whether the script is doing what you want or not.
The console output will also include error messages that may be helpful in debugging the program. For example, if I change line 3 to
pring(number/divisor)
the Python processor will provide an error message, since “pring” is not a valid Python command.
Debugging using V.S. Code
One of the challenging aspects of coding is understanding what is going on in your program when things are not working. V.S. Code has some nice built-in features to help you debug your code. To use those features, go to the Debug menu and select Start Debugging.
When you are in debugging mode, several features become available on the screen. If you click just to the left of the line numbers, you can set breakpoints. A breakpoint is a place in the code where execution will pause in order to give you an opportunity to figure out what is going on at that point. A set breakpoint appears as a red dot to the left of the line number. To start running the code, click on the continue button in the little control panel at the top. The script will run until it reaches the breakpoint and then pause. The line at which the code is stopped is highlighted and the breakpoint dot is surrounded by a yellow pointer. Clicking on the continue button resumes the script at the place where it left off. To remove a breakpoint click on the red dot to toggle it off.
While the code is paused, you can examine the situation of the variables in the program. The pane to the left of the editing pane shows a list of variables and their values. If the variable is a list, the components of the list can be displayed by clicking on the triangle to the left of the variable name. If you want to watch a few particular variables, you can add them to the Watch list at the bottom of the pane. You can also see what the values are of variables by just mousing over them in the main editing pane.
Below the editing pane is a console pane that can display several things. If you have problems with the code itself (such as syntax errors), clicking on the PROBLEMS tab will display them. The DEBUG CONSOLE tab will display any printed output. The TERMINAL tab allows you to issue any command line commands that you could normally type into your operating system console.
Revised 2019-01-18
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License: CC BY 4.0.
Credit: "Vanderbilt Libraries Digital Lab - www.library.vanderbilt.edu"