1.2. Branch review checklist¶
Before you do anything else, watch this: https://www.youtube.com/watch?v=wf-BqAjZb8M&index=2&list=PLdBBfnzuDrjFsiqkVw82l0VAOrUwGAsMn
This section is meant as a guideline for reviewing a pull request. However, it is recommended that you also try to follow these guidelines while working on a new branch, before you make a pull request.
1.2.1. Reviewing protocol¶
126.96.36.199. Step 1. Automatic QA testing¶
When a pull request is made, a bunch of QA tests are executed to detect common problems. This reduces the burden of the reviewers but it is not a replacement for the remainder of the reviewing protocol. We strive to avoid false-positives in the QA scripts such that any error encountered must be fixed in the pull request. This implies that some tests (in Pylint, Pycodestyle, ...) are disabled and must be inspected manually.
If the automatic QA tests do not pass, fix the issues with additional commits to the
branch of the pull request. These commits should appear automatically in the PR. When you
made whitespace errors or when you have to fix author names, rewrite the commits and push
them with the
If you want to run the QA tests locally, this can be done as follows:
Make sure all changes are committed in your feature branch and that the feature branch is checked out.
Optional: if you like, you can build several dependencies of HORTON, using exactly the same version as in the continuous integration (buildbot or Travis-CI):
Run the QA tests:
This is the same script that is used in the continuous integration.
After the QA scripts pass on the build server, your code is ready to be reviewed by a human being.
188.8.131.52. Step 2. Human review¶
The sections below specify a minimum of mindful and mindless criteria that new code must satisfy before it can be merged into the master branch. Some of these were checked by the automatically but a large part still needs to be checked manually.
“A foolish consistency is the hobgoblin of little minds.” (PEP8)
Humans tend to tackle the mindless criteria first, just because they are easier to check. However, when you overemphasize on the mindless ones, you’ll be too busy to see the mindfull issues, even though these are still more important.
To the extent possible, all criteria that have to be checked have a label of the form M?####. These make it easier for reviewers to comment on specific lines of code in PRs.
At least one person other than the author of the PR has to go through the change set and check for conflicts with the criteria below. When issues are found, the author should try to fix these with additional commits. In some cases, issues can only be resolved by rewriting the history of the branch.
184.108.40.206. Step 3. Merging¶
Once the review is completed and all issues are properly solved, someone with write access will check if the reviewing protocol was followed properly. If all is fine, the PR gets merged into the master branch. PRs must be fast-forward merge-able. The author of the PR is responsible for rebasing his/her branch if needed.
1.2.2. Mindful criteria¶
These are criteria that require the reviewer to actually understand the whole change-set. They can’t be formulated in terms of checking just a few lines at a time, which also means such criteria are usually not testable with QA scripts.
220.127.116.11. Atomic commits¶
- MM101 Keep your Git commits as small as possible and avoid that unrelated changes are combined in one commit. This needs to be checked for all commits in a pull request. If needed, a history rewrite is in order.
- MM102 When a commit fixes a bug, include a unit test in the same commit that would break without the fix.
- MM103 When a commit adds a new feature, include a unit tests that validates the implementation.
More background can be found here: http://www.freshconsulting.com/atomic-commits/
18.104.22.168. Unit tests (other than coverage, see below)¶
MM201 The unit tests are divided into two categories: slow and fast ones, where fast means execution time below 1 second. The slow ones are marked with a descriptor in the unit tests source code, while all other ones are assumed to be fast. For example, a slow test is marked as follows:
# In the beginning of the unit test module from nose.plugins.attrib import attr # The slow test @attr('slow') def test_something_slow(): # slow test code
All new code must covered with fast unit tests, which is checked automatically. Also make sure the fast unit tests follow these rules:
- MM201 Code must be unit tested with all possible combinations of arguments and options or with a reasonable subset thereof.
- MM202 Focus on testing the small components, rather than writing tests that depend on a lot of code.
- MM203 Also write unit tests for simple functions.
- MM204 Unit tests should not just run code but also check that the result is in line with expectations.
- MM205 When tests use random numbers, use a fixed random seed, such that at every execution the same random numbers are used. This avoids that a test breaks every now and then. See Writing tests that use random numbers.
- MM206 Avoid tests that just compare the output of a routine with the output from a previous initial version of that routine. Such tests need to be redone each time a bug is found in that routine.
- MM207 Check implementations of analytic derivatives with the tools provided in
HORTON. This is currently the
- MM208 Use
nose.toolsto make unit tests more readable.
22.214.171.124. Code structure¶
- MM301 Whenever using class inheritance, check if it would be more convenient to use composition instead.
Modular code is intuitively obtained through separation of concerns. This means the following:
Different problems are handled in different maximally independent pieces of code (modules).
There are two important parts in this rule:
Different problems. You have to figure what this means in your case. Usually its straightforward, e.g. input/output code is different from actual computation code. You can (and should) obviously make the separation much more fine-grained. For example, different subproblems of a single computation can be separated.
Maximally independent. Modules should depend on each other as little as possible, e.g.:
- The public API of a module has to be as small as possible. If a module has a huge public API, it means that the user must have intimate knowledge of how the module works, which is non-modular.
- The dependence graph of modules should not contain cycles. For example, when A depends on B and B depends on C, then C should not depend on A. It would be fine for A to depend on C.
- Modules should have as little dependencies as possible.
126.96.36.199. Cython is evil¶
- Cython is a constantly evolving language in which one can easily write very unreadable and dirty code. Good coding practices also evolve quickly as Cython is further developed.
- No QA tools (like Pycodestyle, Pydocstyle, Pylint, ...) exist that do some basic QA assurance. Everything has to be checked manually.
Work as follows:
- Either write Python or low-level code (C++, Fortran, ...)
- Only use Cython for wrapping the low-level code.
1.2.3. General mindless criteria¶
188.8.131.52. Miscellaneous (MG01##)¶
- MG0101 Never commit code that breaks unit tests This also means that all tests must pass on every commit. (TODO: this is currently not tested automatically. This would be doable with fast unit tests.)
- MG0102 Never modify or tamper with QA scripts with no good reason.
184.108.40.206. Whitespace errors¶
The following are not allowed (and checked automatically):
- Lines ending with one or more whitespaces.
- Usage of tabs.
- Empty lines at the end of a file.
220.127.116.11. Git commit message format (MG04##)¶
- MG0401 Separate subject from body with a blank line
- MG0402 Limit the subject line to 50 characters
- MG0403 Capitalize the subject line
- MG0404 Do not end the subject line with a period
- MG0405 Use the imperative mood in the subject line
- MG0406 Wrap the body at 72 characters
- MG0407 Use the body to explain what and why vs. how
More background can be found here: http://chris.beams.io/posts/git-commit/
Consider setting the following environment variables such that
vim is used as editor
for the commit messages. It offers syntax highlighting to facilitate writing good commit
export VISUAL=vim export EDITOR="$VISUAL"
18.104.22.168. Units and unit conversion¶
1.2.4. Mindless Python criteria¶
This section first goes over all the criteria that are checked automatically by different programs, listing all enforced rules and exceptions. The last subsection discusses the remaining criteria that have to be checked manually.
The complete list of error codes can be found here: http://pycodestyle.readthedocs.io/en/latest/intro.html#error-codes
The following pycodestyle error codes are disabled.
- By default in pycodestyle (2.0.0):
- E121 (*) continuation line under-indented for hanging indent
- E123 (*) closing bracket does not match indentation of opening bracket’s line
- E126 (*) continuation line over-indented for hanging indent
- E226 (*) missing whitespace around arithmetic operator
- E241 (*) multiple spaces after ‘,’
- W503 (*) line break occurred before a binary operator
- Because they cause undesirable false positives:
- E127 continuation line over-indented for visual indent
- E128 continuation line under-indented for visual indent
All errors caught automatically by the Pydocstyle program must be fixed. Keep in mind that this program does not cover all recommendations in PEP257.
A complete list of error messages can be found here: http://pydocstyle.readthedocs.io/en/latest/error_codes.html
Pydocstyle is executed with the default settings, except that the following is disabled:
- D103: Missing docstring in public function
This is already checked by Pylint and is not enforced for
Pycodestyle cannot yet be configured to ignore test functions. (It can only ignore test
files which is not fine-grained enough for our purposes.)
The complete list of error messages can be found here: https://docs.pylint.org/features.html
The following messages are excluded by default: I0020, I0021, W0704. (It is not clear what these stand for. They are not documented in Pylint.)
The following messages are excluded by default but are activated in our case (related to Python 3): E1601, E1602, E1603, E1604, E1605, E1606, E1607, E1608, W1601, W1602, W1603, W1604, W1605, W1606, W1607, W1608, W1609, W1610, W1611, W1612, W1613, W1614, W1615, W1616, W1617, W1618, W1619, W1620, W1621, W1622, W1623, W1624, W1625, W1626, W1627, W1628, W1629, W1630, W1632, W1633, W1634, W1635, W1636, W1637, W1638, W1639, W1640
The following are excluded because we don’t consider them the be fatal:
- C0103: invalid-name. Invalid %s name “%s”%s Used when the name doesn’t match the regular expression associated to its type (constant, variable, class...).
- I0011: locally-disabled. Used when an inline option disables a message or a messages category.
- W0613: unused-argument. Unused argument %r Used when a function or method argument is not used.
The following is disabled to allow access to the protected members of the (not-so-well-designed) Matrix classes:
- W0212: protected-access. Access to a protected member %s of a client class Used when a protected member (i.e. class member with a name beginning with an underscore) is access outside the class or a descendant of the class where it’s defined.
The following are excluded due false positives:
- E0611: no-name-in-module. No name %r in module %r Used when a name cannot be found in a module.
- E1136: Value ‘%s’ is unsubscriptable emitted when a subscripted value doesn’t support subscription(i.e. doesn’t define __getitem__ method)
- E1101: no-member. %s %r has no %r member Used when a variable is accessed for an unexistent member.
- R0201: no-self-use. Method could be a function Used when a method doesn’t use its bound instance, and so could be written as a function.
- C0411: wrong-import-order. %s comes before %s Used when PEP8 import order is not respected (standard imports first, then third-party libraries, then local imports)
- W0621: Redefining name %r from outer scope (line %s) Used when a variable’s name hide a name defined in the outer scope.
The PyLint settings used by the QA scripts can be found in
of the non-default settings in that file include:
No doc strings are required for unit tests, i.e. functions starting with
Variables that are intentionally unused should get the prefix
_. These are dummy variables. These may be useful when receiving return values that are not used, e.g.
a, _b, c = some_function() # No intent to use _b for dirpath, _dirnames, filenames in os.walk(source_directory): # No intent to use _dirnames in the body of the for loop
The parameters for the design checks are significantly relaxed.
22.214.171.124. Code coverage by (fast) unit tests¶
QA scripts will check if new Python code is touched by unit tests.
126.96.36.199. Mindless Python criteria to be checked manually¶
The following sources were used to compile the list of criteria below, sometimes making verbatim copies:
- PEP8: http://www.python.org/dev/peps/pep-0008/ (version 01-Aug-2013)
- PEP257: http://www.python.org/dev/peps/pep-0257/ (vesion 13-Jun-2001)
- GPSG: https://google.github.io/styleguide/pyguide.html (Revision 2.59)
For every criterion below, the source is mentioned. If the source is prefixed with a twidle, it means that we intentionally deviate from recommendations given in the source. When no source is mentioned, the criteria are specific to HORTON.
- MP0101 (~PEP257) Usage information of a script does not have to be listed in its
module docstring. We use
argparseinstead, which also produces nice usage documentation when the script is called with
- MP0102 (~PEP257) A list of classes, functions, etc in the module docstring is not required as such tables of content are generated automatically by Sphinx.
- MP0103 (PEP257) Module docstrings should start with a short title followed by an empty line. (This is also assumed by scripts that generate the API reference documentation.)
- MP0104 (PEP257) A Module docstring should give some basic background on the module and include some example usage.
- MP0105 (PEP257) Class doc strings should explain the purpose and behavior of a class.
- MP0106 (PEP257) A base class docstring must explain how to implement derived classes.
- MP0107 (PEP257) Function and method docstrings must use an imperative mood in their first line.
- MP0108 Docstrings must be written in RestructedText.
- MP0109 Numpy docstring conventions must be followed.
- MP0101 (~PEP257) Usage information of a script does not have to be listed in its module docstring. We use
MP02## Import conventions
MP0201 NumPy, H5Py, PyPlot and SciPy packages must be imported in the following way:
# Must be on separate lines import numpy as np import h5py as h5 import matplotlib.pyplot as pt # scipy subpackages have to import separately from scipy import whatever
MP0202 (PEP8) Wildcard imports are (only) allowed in two situations:
__init__.pyfiles to republish the API of submodules and subpackages.
In unit tests, one may write:
from horton import *
This tests if everything in HORTON can be properly imported.
MP0203 (PEP8) Put any relevant
__all__specification directly after the imports.
MP0204 (beyond PEP8) Never use relative imports.
MP0205 (PEP8) When importing a class from a class-containing module, it’s usually okay to spell this:
from myclass import MyClass from foo.bar.yourclass import YourClass
If this spelling causes local name clashes, then spell them
import myclass import foo.bar.yourclass
MP03## Naming conventions
MP0301 Use self-explaining variable, function, method, class, module names where reasonable.
MP0302 For integer quantities of something:
MP0303 Loop variable in loop over a number of things:
isomething, e.g. for number of atoms:
for iatom in xrange(natom): # loop content
MP0304 Use plural for arrays, e.g.
MP0305 When using
**constructs in python to allow for an arbitrary number of (keyword) arguments to functions, then always use the names
MP0306 (PEP8) Use
_single_leading_underscoreas a weak “internal use” indicator. E.g.
from M import *does not import objects whose name starts with an underscore.
MP0307 (PEP8) Use
single_trailing_underscore_as to avoid conflicts with Python keyword.
__double_leading_and_trailing_underscore__are “magic” objects or attributes that live in user-controlled namespaces. E.g. __init__ , __import__ or __file__ . Never invent such names; only use them as documented.
MP0309 (PEP8) Modules should have short, all-lowercase names. Underscores can be used in the module name if it improves readability. Python packages should also have short, all-lowercase names, although the use of underscores is discouraged.
MP0310 (PEP8) Class names should normally use the
CapWordsconvention. When abbreviations and acronyms are used in a name, capitalize them, e.g.
MP0311 (PEP8) Use the suffix “Error” on your exception names (if the exception actually is an error).
MP0312 (PEP8) Function, method and variable names should be lowercase, with words separated by underscores as necessary to improve readability.
MP0313 (PEP8) Always use self for the first argument to instance methods.
MP0314 (PEP8) Always use cls for the first argument to class methods.
MP0315 (PEP8) Constants are usually defined on a module level and written in all capital letters with underscores separating words. Examples include
MP04## Cite papers where appropriate. Whenever you add a feature based on a scientific publication, it should be cited properly:
- MP0401 Add an item to the file data/references.bib. Use a lowercase bibtex key, following the lastnameyear convention. Include the doi if possible. (The url field can be used as an alternative if the doi is not available.) Maintain chronological order and alphabetical order within one year.
- MP0402 Cite the references in the HORTON documentation as follows:
- MP0403 Add
log.cite('someref', 'a reason')to the code based on the publication, e.g.
log.cite('marques2012', 'using LibXC, the library of exchange and correlation functionals').
- MP0404 No begging for citations in the output or documentation. Go beyond self-citations. Try to be informative and neutral.
MP05## Code formatting (wrapping, indentation, whitespace, ...)
MP0501 (PEP8) Smart wrapping with parenthesis is preferred over wrapping with a backslash. However, in some cases, this may not be appropriate, like in
MP0502 (PEP8) 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).
MP0503 (PEP8) Use blank lines in functions, sparingly, to indicate logical sections.
MP0504 (PEP8) The first or second line of each Python file must contain
# -*- coding: UTF-8 -*-". (Only needed for Python 2.)
MP0505 (PEP8) If operators with different priorities are used, consider adding whitespace around the operators with the lowest priority(ies). Use your own judgment; however, never use more than one space, and always have the same amount of whitespace on both sides of a binary operator.
i = i + 1 submitted += 1 x = x*2 - 1 hypot2 = x*x + y*y c = (a+b) * (a-b)
i=i+1 submitted +=1 x = x * 2 - 1 hypot2 = x * x + y * y c = (a + b) * (a - b)
MP0506 (PEP8) Don’t use spaces around the
=sign when used to indicate a keyword argument or a default parameter value.
def munge(input: AnyStr): def munge(sep: AnyStr = None): def munge() -> AnyStr: def munge(input: AnyStr, sep: AnyStr = None, limit=1000):
def munge(input: AnyStr=None): def munge(input:AnyStr): def munge(input: AnyStr)->PosInt:
MP0507 (PEP8) Do use spaces around the = sign of an annotated function definition. Additionally, use a single space after the : , as well as a single space on either side of the -> sign representing an annotated return value.
MP0508 (GPSG) Every file should contain license boilerplate.
MP0509 Scripts (and scripts only) should have
#!/usr/bin/env pythonas their first line.
MP06## Follow the PEP8 rules for comments, except for Strunk and White if you know better: https://www.python.org/dev/peps/pep-0008/#comments. (See also block comments and inline comments.) TODO: make list of bullet points instead of just linking to PEP8
- MP0701 (PEP8, GPSG) Do not define public
set_*methods in a class that involve litte computation. Make these methods non-public (prefix with underscore) and wrap them in a property instead. If these methods involve significant computation, they are fine as a method but try to find a better name
- MP0702 Follow the PEP8 rules given here: https://www.python.org/dev/peps/pep-0008/#public-and-internal-interfaces TODO: make list of bullet points instead of just linking to PEP8
- MP0703 (PEP8) All of the following applies, except that we completely dissalow name mangling: https://www.python.org/dev/peps/pep-0008/#designing-for-inheritance TODO: make list of bullet points instead of just linking to PEP8
- MP0701 (PEP8, GPSG) Do not define public
MP08## Exception handling
MP0801 (PEP8) Derive exceptions from
MP0802 (PEP8) When raising an exception in Python 2, use raise
ValueError('message')instead of the older form raise
ValueError, 'message'. The latter form is not legal Python 3 syntax.
MP0803 (PEP8) When catching exceptions, mention specific exceptions whenever possible instead of using a bare
MP0804 (PEP8) When binding caught exceptions to a name, prefer the explicit name binding syntax added in Python 2.6:
try: process_data() except Exception as exc: raise DataProcessingFailedError(str(exc))
This is the only syntax supported in Python 3, and avoids the ambiguity problems associated with the older comma-based syntax.
MP0805 (PEP8) Additionally, for all try/except clauses, limit the try clause to the absolute minimum amount of code necessary. This avoids masking bugs.
try: value = collection[key] except KeyError: return key_not_found(key) else: return handle_value(value)
try: # Too broad! return handle_value(collection[key]) except KeyError: # Will also catch KeyError raised by handle_value() return key_not_found(key)
MP0806 (GPSG) When the built-in exceptions seem inappropriate or too vague (e.g. like
RuntimeError), modules should defined their own exceptions. These exceptions must be defined in the module where they are used.
MP09## Boolean expressions, implicit True/False
MP0905 (PEP8) Don’t compare boolean values to True or False using == .
# Yes: if greeting: # No: if greeting == True: # Worse: if greeting is True:
MP0906 (~PEP8, ~GPSG) For sequences, (strings, lists, tuples), DO NOT use the fact that empty sequences are false.
This is recommended in PEP8 but it is not very readable as one has to know the type of
seqto figure out what is going on:
if not seq: if seq:
Not good either:
if len(seq): if not len(seq):
Recommended explicit form:
if len(seq) > 0: if len(seq) == 0:
MP0906 (GPSG) When handling integers, implicit false may involve more risk than benefit (i.e., accidentally handling None as 0).
MP9901 (PEP8) When a resource is local to a particular section of code, use a
withstatement to ensure it is cleaned up promptly and reliably after use.
MP9902 (PEP8) Be consistent in return statements. Either all return statements in a function should return an expression, or none of them should. If any return statement returns an expression, any return statements where no value is returned should explicitly state this as
return None, and an explicit return statement should be present at the end of the function (if reachable).
MP9903 (PEP8) Use string methods instead of the
MP9904 (PEP8) Use
.endswith()instead of string slicing to check for prefixes or suffixes.
MP9905 (GPSG) Use default iterators and operators for types that support them, like lists, dictionaries, and files. The built-in types define iterator methods, too. Prefer these methods to methods that return lists, except that you should not mutate a container while iterating over it.
for key in adict: ... if key not in adict: ... if obj in alist: ... for line in afile: ... for k, v in dict.iteritems(): ...
for key in adict.keys(): ... if not adict.has_key(key): ... for line in afile.readlines(): ...
MP9906 (GPSG) Avoid unreadable functional programming constructs, e.g. because they do not fit on one or two lines. This includes list comprehensions, lambda functions and inline conditionals.
MP9907 (GPSG, PEP8) If a class inherits from no other base classes, explicitly inherit from object. This also applies to nested classes.
MP9908 (GPSG) Your code should always check if
__name__ == '__main__'before executing your main program so that the main program is not executed when the module is imported. Just call the
mainfunction in this if clause instead of adding a lot of code in the global scope.
def main(): # ... if __name__ == '__main__': main()
1.2.5. Mindless C++ criteria¶
All C++ code should make use of the C++11 standard. Automatic checks are only applied to manually written C++ code. Autogenerated code is excluded from such tests.
CPPCheck is executed with all checks enabled and with the C++11 flag. The following exceptions are added due to false positives:
All other errors must be fixed.
CPPLint is executed with the default settings, except that the maximum line length is set to 100. All errors must be fixed.
188.8.131.52. Manual checks¶
The following points should be checked manually. These are taken from the Google C++ Style Guide (GCSG). See https://google.github.io/styleguide/cppguide.html
MC00## Header files
- MC0001 (GCSG) #define guard
All header files should have #define guards to prevent multiple inclusion. The
format of the symbol name should be
- MC0004 (GCSG) Names and Order of Includes
Use standard order for readability and to avoid hidden dependencies: Related header,
C library, C++ library, other libraries’
.h, your project’s
- MC0001 (GCSG) #define guard All header files should have #define guards to prevent multiple inclusion. The format of the symbol name should be
- MC0202 (GCSG) Copyable and Movable Types Support copying and/or moving if it makes sense for your type. Otherwise, disable the implicitly generated special functions that perform copies and moves.
- MC0210 (GCSG) Declaration Order Use the specified order of declarations within a class: public: before private:, methods before data members (variables), etc.
- MC0300 (GCSG) Parameter Ordering
- MC0302 (GCSG) Reference Arguments All parameters passed by reference must be labeled const.
- MC0401 (GCSG) Variable-Length Arrays and alloca() We do not allow variable-length arrays or alloca().
- MC0407 (GCSG) Preincrement and Predecrement
Use prefix form (
++i) of the increment and decrement operators with iterators and other template objects.
- MC0408 (GCSG) Use of const
constwhenever it makes sense. With C++11,
constexpris a better choice for some uses of
- MC0503 (GCSG) Variable Names
The names of variables and data members are all lowercase, with underscores between
words. Data members of classes (but not structs) additionally have trailing
underscores. For instance:
- MC0503 (GCSG) Variable Names The names of variables and data members are all lowercase, with underscores between words. Data members of classes (but not structs) additionally have trailing underscores. For instance:
184.108.40.206. API documentation¶
QA scripts will test if C++ source code documentation is missing.
220.127.116.11. Code coverage¶
For the moment, the C++ code is not included in the coverage analysis. It wasn’t possible
gcov to work on the C++ extensions.
1.2.6. Mindless Cython criteria¶
All Python criteria must be followed but nothing can be tested automatically. You have to check everything manually. Because this is horribly inconvenient, the amount of Cython code should be kept to a minimum.
In addition to the Python criteria, also use the following conventions:
MY0001 Pointers to NumPy array data should be accessed as follows:
cdef double* pointer pointer = &array # for a 1D array pointer = &array[0, 0] # for a 2D array pointer = &array[0, 0, 0] # for a 3D array # etc.
MY0002 NumPy should be imported in Cython in a specific way. (See https://github.com/cython/cython/wiki/tutorials-numpy#c-api-initalization)
import numpy as np cimport numpy as np np.import_array() # Other import and cimport lines should be put below.
18.104.22.168. Comments (MG02##)¶
Python doc strings are great for documenting the API, but they are not sufficient to document the internals of code. When someone wants to understand your code, additional help is highly appreciated in the form of comments. At least include the following information:
MG0201 A dictionary of local variables when variable names that do not speak for themselves. Such a dictionary can be written at the level of a single function or method, but it also makes sense at a class or even module level when certain variable names are consistently throughout. (Such consistency is preferable where possible.)
MG0202 When using variable names that correspond to symbols from equations in some paper, add a reference to that paper and the relevant equations. Also explicitly list the variables that correspond to symbols in the reference.
MG0203 Explain the role of groups of lines of source code in comments. Usually a single line does not need to be explained with a comment, unless it looks really cryptic. (Such cryptic statements should rather be avoided.) For example, it does not make sense to write the following comment because the Python code speaks for itself: