Commit 9110fb4a authored by Santi Raffa's avatar Santi Raffa Committed by Petr Pudlak
Browse files

Add code style document to documentation

The Ganeti code style has been stored on the project wiki at:

This commit combines the two pages into an .rst file with minimal
formatting and language changes. Note that the style guide introduced
in this commit does not fit the code base in a number of ways,

* Some Haskell files have lines longer than 78 characters
* Some Haskell files have trailing whitespace
* Some Python docstring initial sentences lack punctuation at the end

The decision to either change the offending lines to fit the guidelines,
to change the guidelines to fit the codebase or to simply ignore the
discrepancies is left for other commits to solve.
Signed-off-by: default avatarSanti Raffa <>
Signed-off-by: default avatarPetr Pudlak <>
Reviewed-by: default avatarPetr Pudlak <>
parent ed748771
......@@ -555,6 +555,7 @@ docinput = \
doc/design-upgrade.rst \
doc/design-virtual-clusters.rst \
doc/design-x509-ca.rst \
doc/dev-codestyle.rst \
doc/devnotes.rst \
doc/glossary.rst \
doc/hooks.rst \
Code style guide
.. highlight:: python
These are a few guidelines for Ganeti code and documentation.
In simple terms: try to stay consistent with the existing code. `PEP 8`_ says:
.. _PEP 8:
A style guide is about consistency. Consistency with this style guide is
important. Consistency within a project is more important. Consistency
within one module or function is most important.
.. note::
You might also want to take a look at the `Google style guide`_, since we
have some things in common with it.
.. _Google style guide:
In general, always indent using two (2) spaces and don't use tabs.
The two spaces should always be relative to the previous level of indentation,
even if this means that the final number of spaces is not a multiple of 2.
When going on a new line inside an open parenthesis, align with the content of
the parenthesis on the previous line.
Valid example::
v = (somevalue,
list_elem, # 7 spaces, but 2 from the previous indentation level
Formatting strings
Always use double quotes (``""``), never single quotes (``''``), except for
existing code. Examples for formatting strings::
var = "value"
# Note: The space character is always on the second line
var = ("The quick brown fox jumps over the lazy dog. The quick brown fox"
" jumps over the lazy dog. The quick brown fox jumps over the lazy"
" dog.")
fn("The quick brown fox jumps over the lazy dog. The quick brown fox jumps"
" over the lazy dog.")
("The quick brown fox jumps over the lazy dog. The quick brown fox"
" jumps over the lazy dog. The quick brown fox jumps over the lazy"
" dog."))
Don't format strings like this::
# Don't use single quotes
var = 'value'
# Don't use backslash for line continuation
var = "The quick brown fox jumps over the lazy dog. The quick brown fox"\
" jumps over the lazy dog."
# Space character goes to the beginning of a new line
var = ("The quick brown fox jumps over the lazy dog. The quick brown fox "
"jumps over the lazy dog. The quick brown fox jumps over the lazy "
Formatting sequences
Built-in sequence types are list (``[]``), tuple (``()``) and dict (``{}``).
When splitting to multiple lines, each item should be on its own line and a
comma must be added on the last line. Don't write multiline dictionaries in
function calls, except when it's the only parameter. Always indent items by
two spaces.
# Short lists
var = ["foo", "bar"]
var = ("foo", "bar")
# Longer sequences and dictionary
var = [
var = {
"key": func(),
"otherkey": None,
# Multiline tuples as dictionary values
var = {
("long value taking the whole line, requiring you to go to a new one",
# Function calls
var = frozenset([1, 2, 3])
var = F({
"xyz": constants.XYZ,
"abc": constants.ABC,
# Wrong
F(123, "Hello World",
{ "xyz": constants.XYZ })
We consider tuples as data structures, not containers. So in general please
use lists when dealing with a sequence of homogeneous items, and tuples when
dealing with heterogeneous items.
Passing arguments
Positional arguments must be passed as positional arguments, keyword arguments
must be passed as keyword arguments. Everything else will be difficult to
# Function signature
def F(data, key, salt=None, key_selector=None):
# Yes
F("The quick brown fox", "123456")
F("The quick brown fox", "123456", salt="abc")
F("The quick brown fox", "123456", key_selector="xyz")
F("The quick brown fox", "123456", salt="foo", key_selector="xyz")
# No: Passing keyword arguments as positional argument
F("The quick brown fox", "123456", "xyz", "bar")
# No: Passing positional arguments as keyword argument
F(salt="xyz", data="The quick brown fox", key="123456", key_selector="xyz")
.. note::
`PEP 257`_ is the canonical document, unless epydoc overrules it (e.g. in how
to document the type of an argument).
For docstrings, the recommended format is epytext_, to be processed via
epydoc_. There is an ``apidoc`` target that builds the documentation and puts it
into the doc/api subdir. Note that we currently use epydoc version 3.0.
.. _PEP 257:
.. _epytext:
.. _epydoc:
Note that one-line docstrings are only accepted in the unittests.
Rules for writing the docstrings (mostly standard Python rules):
* the docstring should start with a sentence, with punctuation at the end,
summarizing the the aim of what is being described. This sentence cannot be
longer than one line
* the second line should be blank
* afterwards the rest of the docstring
* special epytext tags should come at the end
* multi-line docstrings must finish with an empty line
* do not try to make a table using lots of whitespace
* use ``L{}`` and ``C{}`` where appropriate
Here's an example::
def fn(foo, bar):
"""Compute the sum of foo and bar.
This functions builds the sum of foo and bar. It's a simple function.
@type foo: int
@param foo: First parameter.
@type bar: float
@param bar: The second parameter. This line is longer
to show wrapping.
@rtype: float
@return: the sum of the two numbers
return foo + bar
Some rules of thumb which should be applied with good judgement on a case-to-
case basis:
* If the meaning of parameters is already obvious given its name and the
methods description, don't document it again. Just add a ``@type`` tag.
* Refer to the base methods documentation when overwriting methods. Only
document more if it applies to the current subclass only, or if you want to
clarify on the meaning of parameters for the special subclass.
Rules for classes and modules
As `PEP 257`_ says, the docstrings of classes should document their attributes
and the docstrings of modules should shortly document the exported
See for example the pydoc output for the ``os`` or ``ConfigParser`` standard
.. highlight:: haskell
The most important consideration is, as usual, to stay consistent with the
existing code.
As there's no "canonical" style guide for Haskell, this code style has been
inspired from a few online resources, including the style guide for the
`Snap framework`_, `this style guide`_ and `this other style guide`_.
.. _Snap framework:
.. _this style guide:
.. _this other style guide:
Use ordinary, non-`literate`_ Haskell ``.hs`` files.
.. _literate:
Use proper copyright headers, and proper Haddock style documentation headers::
{-| Short module summary.
Longer module description.
Copyright (C) ...
This program is free software ...
If there are module-level pragmas add them right at the top, before the short
Imports should be grouped into the following groups and inside each group they
should be sorted alphabetically:
1. standard library imports
2. third-party imports
3. local imports
It is allowed to use qualified imports with short names for:
* standard library (e.g. ``import qualified Data.Map as M``)
* local imports (e.g. ``import qualified Ganeti.Constants as C``), although
this form should be kept to a minimum
Use only spaces, never tabs. Indentation level is 2 characters. For Emacs,
this means setting the variable ``haskell-indent-offset`` to 2.
Line length should be at most 78 chars, and 72 chars inside comments.
Use indentation-based structure, and not braces/semicolons.
.. note::
Special indendation of if/then/else construct
For the ``do`` notation, the ``if-then-else`` construct has a non-intuitive
behaviour. As such, the indentation of ``if-then-else`` (both in ``do``
blocks and in normal blocks) should be as follows::
if condition
then expr1
else expr2
i.e. indent the then/else lines with another level. This can be accomplished
in Emacs by setting the variable ``haskell-indent-thenelse`` to 2 (from the
default of zero).
If you have more than one line of code please newline/indent after the "=". Do
`not` do::
f x = let y = x + 1
in y
Instead do::
f x =
let y = x + 1
in y
or if it is just one line::
f x = x + 1
Multiline strings
Multiline strings are created by closing a line with a backslash and starting
the following line with a backslash, keeping the indentation level constant.
Whitespaces go on the new line, right after the backslash.
longString :: String
longString = "This is a very very very long string that\
\ needs to be split in two lines"
Data declarations
.. warning::
Note that this is different from the Python style!
When declaring either data types, or using list literals, etc., the columns
should be aligned, and for lists use a comma at the start of the line, not at
the end. Examples::
data OpCode = OpStartupInstance ...
| OpShutdownInstance ...
| ...
data Node = Node { name :: String
, ip :: String
, ...
myList = [ value1
, value2
, value3
The choice of whether to wrap the first element or not is up to you; the
following is also allowed::
myList =
[ value1
, value2
White space
Like in Python, surround binary operators with one space on either side. Do no
insert a space after a lamda::
-- bad
map (\ n -> ...) lst
-- good
foldl (\x y -> ...) ...
Use a blank line between top-level definitions, but no blank lines between
either the comment and the type signature or between the type signature and
the actual function definition.
.. note::
Ideally it would be two blank lines between top-level definitions, but the
code only has one now.
As always, no trailing spaces. Ever.
Spaces after comma
Instead of::
("a", "b")
Functions should be named in mixedCase style, and types in CamelCase. Function
arguments and local variables should be mixedCase.
When using acronyms, ones longer than 2 characters should be typed capitalised,
not fully upper-cased (e.g. ``Http``, not ``HTTP``).
For variable names, use descriptive names; it is only allowed to use very
short names (e.g. ``a``, ``b``, ``i``, ``j``, etc.) when:
* the function is trivial, e.g.::
sum x y = x + y
* we talk about some very specific cases, e.g.
iterators or accumulators in folds::
map (\v -> v + 1) lst
* using ``x:xs`` for list elements and lists, etc.
In general, short/one-letter names are allowed when we deal with polymorphic
values; for example the standard map definition from Prelude::
map :: (a -> b) -> [a] -> [b]
map _ [] = []
map f (x:xs) = f x : map f xs
In this example, neither the ``a`` nor ``b`` types are known to the map
function, so we cannot give them more explicit names. Since the body of the
function is trivial, the variables used are longer.
However, if we deal with explicit types or values, their names should be
.. todo: add a nice example here.
Finally, the naming should look familiar to people who just read the
Prelude/standard libraries.
Naming for updated values
.. highlight:: python
Since one cannot update a value in Haskell, this presents a particular problem
on the naming of new versions of the same value. For example, the following
code in Python::
def failover(pri, sec, inst):
.. highlight:: haskell
becomes in Haskell something like the following::
failover pri sec inst =
let pri' = removePrimary pri inst
pri'' = addSecondary pri' inst
sec' = removeSecondary sec inst
sec'' = addPrimary sec' inst
in (pri'', sec'')
When updating values, one should add single quotes to the name for up to three
new names (e.g. ``inst``, ``inst'``, ``inst''``, ``inst'''``) and otherwise
use numeric suffixes (``inst1``, ``inst2``, ``inst3``, ..., ``inst8``), but
that many updates is already bad style and thus should be avoided.
Type signatures
Always declare types for functions (and any other top-level bindings).
If in doubt, feel free to declare the type of the variables/bindings in a
complex expression; this usually means the expression is too complex, however.
Similarly, provide Haddock-style comments for top-level definitions.
Parentheses, point free style
Prefer the so-called point-free style to extra parentheses::
-- bad
let a = f ( g ( h x) )
-- better
let b = f $ g $ h x
-- best
let c = f . g . h $ x
Language features
It is recommended to keep the use of extensions to a minimum, so that the code
can be understood even if one is familiar with just Haskel98/Haskell2010. That
said, some extensions are very common and useful, so they are recommended:
* `Bang patterns`_: useful when you want to enforce strict evaluation (and better
than repeated use of ``seq``)
* CPP: a few modules need this in order to account for configure-time options;
don't overuse it, since it breaks multi-line strings
* `Template Haskell`_: we use this for automatically deriving JSON instances and
other similar boiler-plate
.. _Bang patterns:
.. _Template Haskell:
Such extensions should be declared using the ``Language`` pragma::
{-# Language BangPatterns #-}
{-| This is a small module... -}
Always use proper sentences; start with a capital letter and use punctuation
in top level comments::
-- | A function that does something.
f :: ...
For inline comments, start with a capital letter but no ending punctuation.
Furthermore, align the comments together with a 2-space width from the end of
the item being commented::
data Maybe a = Nothing -- ^ Represents empty container
| Just a -- ^ Represents a single value
The comments should be clear enough so that one doesn't need to look at the
code to understand what the item does/is.
Use ``-- |`` to write doc strings rather than bare comment with ``--``.
We generate the API documentation via Haddock, and as such the comments should
be correct (syntax-wise) for it. Use markup, but sparingly.
We use hlint_ as a lint checker; the code is currently lint-clean, so you must
not add any warnings/errors.
.. _hlint:
Use these two commands during development::
make hs-apidoc
make hlint
QuickCheck best practices
If you have big type that takes time to generate and several properties to
test on that, by default 500 of those big instances are generated for each
property. In many cases, it would be sufficient to only generate those 500
instances once and test all properties on those. To do this, create a property
that uses ``conjoin`` to combine several properties into one. Use
``printTestCase`` to add expressive error messages. For example::
prop_myMegaProp :: myBigType -> Property
prop_myMegaProp b =
[ printTestCase
("Something failed horribly here: " ++ show b) (subProperty1 b)
, printTestCase
("Something else failed horribly here: " ++ show b)
(subProperty2 b)
, -- more properties here ...
subProperty1 :: myBigType -> Bool
subProperty1 b = ...
subProperty2 :: myBigType -> Property
subProperty2 b = ...
Maybe Generation
Use ``genMaybe genSomething`` to create ``Maybe`` instances of something
including some ``Nothing`` instances.
Use ``Just <$> genSomething`` to generate only ``Just`` instances of
String Generation
To generate strings, consider using ``genName`` instead of ``arbitrary``.
``arbitrary`` has the tendency to generate strings that are too long.
......@@ -116,6 +116,11 @@ the installed and developed versions are very similar, and/or if
PYTHONPATH is customised correctly). As such, in general it's
recommended to use a "clean" machine for ganeti development.
Style guide
Please adhere to the :doc:`dev-codestyle` while writing code for Ganeti.
Haskell development notes
......@@ -137,6 +137,7 @@ Draft designs
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