Code Quality Assistance Tips and Tricks, or How to Make Your Code Look Pretty?

What this tutorial is about

This tutorial aims to walk you step by step through creating source code in a Python project, with the use of PyCharm's code intelligence features. You will see how PyCharm helps keep your source code in perfect shape, with proper indentations, spaces, imports etc. - actually, you'll see that PyCharm itself is a code quality tool.

Python programming is out of scope of this tutorial. To learn more about the Python language, please refer to the official website.

Before you start

Make sure that:

• You are working with PyCharm version 5.0 or later. If you still do not have PyCharm, download it from this page. To install PyCharm, follow the instructions, depending on your platform. Refer to the product documentation for details.

• You have created a Python project (File|New Project...). Refer to the product documentation for details.

• You have created two directories src and test_dir (File|New or Alt+Insert).

• You have added Python files to the src and test_dir directories of your project(File|New or Alt+Insert). To learn about creating files, refer to the section Populating Projects.

Highliting code style violations

Create a new Python file src/Solver.py Alt+Insert. The created file immediately opens for editing. The file by default has no contents - this is because the file Solver.py is created by a file template that (in the case of Python files) contains just nothing.

Next, start typing the keyword class. When you just start typing, PyCharm immediately shows the suggestion list to complete your code:

(Refer to Code Completion page of the product documentation for details.)

The red curve marks the next expected entry - in this case, this is the expected identifier. Enter the class name Solver. The red curve moves after the class name. If you hover your mouse pointer over this curve, you see the error description ("Colon expected"). Also, mind the red error stripe in the right gutter - it also marks the same error:

OK, type the colon, and press Enter. According to the Python code style, the next statement is indented. If by chance you press space after Enter, you will thus violate the code style settings.

Tuning the PEP8 inspections

However, by default these violation are but weak warnings, and as such, are not visible. So, at first, let's raise their importance. Click on the main toolbar, on the Inspections page of the Settings dialog, type PEP8 to find all PEP8-related inspections, and from the Severity drop-down list, choose Warning:

Apply changes and close the dialog. Now let's return to our source code.

Tracking PEP8 rules

Now PyCharm shows its best! It stands on guard to protect your code style integrity. You immediately note that indented space is highlighted, and, when you type the next statement, for example, def demo(self,a,b,c):, PyCharm will show the message from the PEP8 inspection:

So, as you can see, PyCharm supports PEP8 as the official Python style guide. If you explore the list of inspections (Ctrl+Alt+S - Inspections), you will see that PyCharm launches the pep8.py tool on your code, and pinpoints the code style violations.

Code inspections and their settings

Btw, look at the Inspections more attentively. If you have just opened this page, you see the default inspection profile with the default settings: it means that the inspections apply to all the sources of the current project.

Let's try to customize this profile for two different scopes:

• In the Test scope, the spelling errors should be marked as typos (green)

• In the Production scope, the spelling errors should be marked as errors (red) - can we actually produce code with typos?

This is how it's done...

Creating scopes

First, let's define the two scopes. To do that, click on the main toolbar, in the Settings dialog box expand the node Appearance and Behavior, open the page Scopes. Then click and choose scope type Local.

In the Add New Scope dialog box, type the scope name (Test), and then, in the project tree, choose the directory to be included in the Test scope, test_dir. Note that the Pattern field is filled in automatically, as you include the directory:

Repeat this process to create the Production scope.

Creating inspection profile with these scopes

Next, let's create a copy of the default profile (though this profile is editable... just to be on the safe side):

and give it a new name, for example, MyProjectProfile. This new profile is a copy of the default one, and has the same set of inspections.

With this new profile selected, let's locate the Spelling inspection and change it. To find the Spelling inspection (we've already done it before), just type spel in the search area.

What's next? Click In All Scopes button and select the Test scope from the list; repeat same for the Production scope

In the scope "Test", the inspection severity is left as-is (a typo); however, the scope "Production" we'll choose "Error" from the list of severities:

Mind the color code of inspections. They are shown black if unchanged. If they are blue, then it means that they have been changed.

Apply changes and close the dialog...

So, the modified inspection profile is ready. Its name is Project Default (copy), and it has different settings for the Spelling inspection in the Test and Production scopes. Next, let's inspect code against this profile. To do that, choose Code|Inspect Code on the main menu, and in the dialog box, choose the desired profile and scope:

Do it twice - for Test and Production scopes (if you want to preserve inspection results for further examination and sharing, you can export them). Explore results:

Highlighting errors

Besides coding style violations, PyCharm highlights the other errors too, depending on the selected profile.

For example, if your inspection profile includes Python inspection Unresolved references, and you use a symbol that not yet has been imported, PyCharm underlines the unresolved reference and suggests to add import statement:

Refer to the product documentation.

Generating source code

PyCharm provides lots of possibilities to automatically generate code. You can explore the auto-generation features in the product documentation. Let's explore the main code generation procedures. To do that, just delete all contents of the file Solver.py, and start from scratch.

First, create an instance of a class:

Next, press Alt+Enter and choose the intention action Create class 'Solver':

Great! PyCharm has stubbed out a class:

Next, let's add a method to the class instance. To do that, type a dot after class instance, and then type the method name. This method does not yet exist, and PyCharm suggests to create one:

Let's do some manual work - type the source code. When it comes to calculate the discriminant, we have to extract a square root. There is a dedicated function sqrt in the library math, but it is not yet imported. OK, let's type it anyway, and see how PyCharm copes with it. Press Alt+Enter and choose Import 'math':

So, we've come to the source code like this:

import math
class Solver(object):
            def demo(self,a,b,c):
                d = b ** 2 - 4 * a * c
                disc = math.sqrt(d)
                root1 = (- b + disc) / (2 * a)
                root2 = (- b - disc) / (2 * a)
                print (root1, root2)
                return root1, root2

However, it lacks some significant analysis. We'd like to analyze the radicand d. If it is zero or positive, then the discriminant and the equation roots will be calculated; when the radicand is negative, let's raise an exception. How PyCharm will help complete this task?

Let's surround a block of code with if construct. Select the statements to be completed, when d is non-negative, and press Ctrl+Alt+T (or choose Code | Surround with on the main menu):

Select if option from the suggestion list. As you see, PyCharm automatically adds if True: and indents the selected lines:

We are not at all interested in a boolean expression, so let's change the selected True to d >= 0. Next, place the caret at the end of the last line, and press Enter. The caret rests on the next line, with the same indentation as the if statement; type the else: clause here, and see PyCharm reporting about the expected indentation:

When you press Enter again, the caret rests at the indented position. Here you can type the exception expression, using PyCharm's powerful automatic code completion:

Reformatting code

Let's look again at our Solver.py file. Its right gutter shows yellow stripes. When you hover your mouse pointer over a stripe, PyCharm shows the description of the corresponding problem in the code:

The good news is that they are but warnings, and won't affect the results. Bad news is they are too numerous to fix each one by one. Is it possible to make the source code nice and pretty without much fuss?

PyCharm says - yes. This is the code reformatting feature. So let's try to change formatting of the entire file. To do that, press Ctrl+Alt+L (or choose Code | Reformat Code on the main menu):

Look at the code now - the PEP8-related drawbacks are all gone.

Note that you can define formatting rules yourself. To do that, open the code style settings, select language (in this case, Python), and make the necessary changes:

Adding documentation comments

OK, formatting is fixed now, but there are still some stripes left. The inevitable yellow light bulb shows the possibility to add a docstring comment:

Choose this suggestion and see the docstring comment for a certain parameter added:

Note that you have to select the checkbox Insert type placeholders in documentation comment strings in the Smart Keys page of the Editor settings:

There are several docstring formats, and the documentation comments are created in the format, which you have selected in the Python Integrated Tools page. If you so wish, you can change the docstring format to, say, Epytext or plain text.

Type hinting

The documentation comments can be used to define the expected types of parameters, return values, or local variables. Why do we need it all? For example, we'd like to keep under control the types of parameters we pass to the demo() method. To do that, let's add the corresponding information to the documentation comment (By the way, mind code completion in the documentation comments!):

Next, when you look at the method invocation, you see that the wrong parameter is highlighted by the PyCharm's inspection Type Checker:

Learn more about type hinting in the PyCharm documentation.