Flake8 - check for blank lines around statements

PEP 8 recommends to use blank lines only to separate logical sections:

Extra blank lines may be used (sparingly) to separate groups of related functions. Blank lines may be omitted between a bunch of related one-liners (e.g. a set of dummy implementations).

Use blank lines in functions, sparingly, to indicate logical sections.

However, adding blank lines before and after compound statements (e.g. if/else block) as well as some simple statements (e.g. return) might improve code readability which is otherwise hindered despite syntax highlighting that modern code editors provide, as demonstrated in the following example:

if 1 == 1:
    print(1)
for n in [2, 3]:
    print(n)
else:
    print(4)

...where it might not be immediately apparent that this is not one if/else statement but an if statement followed by a for/else statement.

This Flake8 plugin therefore checks for a blank line before/after each statement as long as it's not the first/last line of code within a module and not the first/last statement within a compound statement.

Requirements

• Python >= 3.8.1
• Flake8 >= 3.8.0

Installation

Pip

pip install flake8-bas

Poetry

poetry add flake8-bas

Statements and their error codes

The statements are split into different categories based on whether they are simple statements or compound statements, and whether an error occurs between two statements of the same type or not. This allows you to filter entire groups using BAS and the first digit, e.g. BAS2.

Error types:

• Before Error - missing line before a statement as long as the preceding element is not a statement of the same type.
• After Error - missing line after a statement as long as the element that follows is not a statement of the same type.
• Sibling Error - missing line between two or more consecutive statements of the same type.

BAS1xx/BAS2xx/BAS3xx: Simple statements

Simple statements, excluding expressions and assignments, which are technically statements as well.

Statement	Before Error	After Error	Sibling Error
assert	BAS101	BAS201	BAS301
break	BAS102	BAS202	BAS302
continue	BAS103	BAS203	BAS303
del	BAS104	BAS204	BAS304
global	BAS105	BAS205	BAS305
import	BAS106	BAS206	BAS306
from import	BAS107	BAS207	BAS307
nonlocal	BAS108	BAS208	BAS308
pass	BAS109	BAS209	BAS309
raise	BAS110	BAS210	BAS310
return	BAS111	BAS211	BAS311
yield	BAS112	BAS212	BAS312
yield from	BAS113	BAS213	BAS313

Note: Some of these errors shouldn't occur (e.g. return followed by another return) because having consecutive siblings of those types does not make sense, but the plugin would raise these errors anyway.

BAS5xx/BAS6xx/BAS7xx: Compound statements

Statement	Before Error	After Error	Sibling Error
class	BAS501	BAS601	BAS701
def	BAS502	BAS602	BAS702
async def	BAS503	BAS603	BAS703
for	BAS504	BAS604	BAS704
async for	BAS505	BAS605	BAS705
if	BAS506	BAS606	BAS706
match	BAS507	BAS607	BAS707
try	BAS508	BAS608	BAS708
while	BAS509	BAS609	BAS709
with	BAS510	BAS610	BAS710
async with	BAS511	BAS611	BAS711

Overlapping errors

The extension produces overlapping errors, that is two statements of different types following each other would produce one "before" error and one "after" error pointing to the same line of code:

a = 1

global a
del a

This would result in two errors for line 4:

./file.py:4:1: BAS205 missing blank line after "global" statement
./file.py:4:1: BAS104 missing blank line before "del" statement

However, two statements of the same type would produce only one "sibling" error.

Configuration

The plugin checks for blank lines around every statement. There are no custom configuration options. Instead, you could simply ignore some errors. This system has benefits as well as drawbacks.

The benefit is that you could take advantage of Flake8's ignore and per-file-ignores (flake8 >= 3.7.0) config options and have a different behaviour applied to each set of files:

[flake8]
ignore = BAS3
per-file-ignores =
    app/*: BAS10, BAS110, BAS20, BAS210
    tests/*: BAS1, BAS2

The drawback is that there are no sane defaults and you would inevitably need to exclude some errors, either because they are undesirable, make little sense, or the same/conflicting checks might already be applied by another plugin (e.g. checks by flake8-import-order) or should be handled by other (formatting) tools (e.g. black).

Recommended exclusions

Only compound statements plus return and yield would raise errors:

[flake8]
ignore = BAS10, BAS110, BAS20, BAS210, BAS30, BAS310

All simple statements excluded

Only compound statements would raise errors:

[flake8]
ignore = BAS1, BAS2, BAS3