Add imports to style guide

Author: Mauko Quiroga

STYLEGUIDE.md

@@ -0,0 +1,84 @@
+# OpenFisca's Python Style Guide
+
+Arguments over code style and formatting are the bread and butter of most open-source projets out there, including OpenFisca.
+
+To avoid this, we have a style guide, that is a set or arbitrary but consistent conventions about how code should be written, for contributors and maintainers alike.
+
+## Notice
+
+### About this style guide
+
+The present style guide is a work in progress.
+
+It largely follows [Python Foundation's](https://www.python.org/dev/peps/pep-0008/), [NumPy's](https://numpydoc.readthedocs.io/en/latest/format.html) and [Google's](https://google.github.io/styleguide/pyguide.html), but it has a lot of her own as well.
+
+### Contributing
+
+Please refer whenever possible to this style guide both for your contributions and your reviews.
+
+If the style in question is not present and contentious, do not hesitate to include an addition to this guide within your proposal or review.
+
+## Imports
+
+1. In general, use import statements for entire namespaces and modules, rather than for classes and functions. Are exempt of this rule the `typing` module, NumPy's `typing` module, and `openfisca_core` module and submodules (but only in the case of class imports).
+
+2. In general, use absolute import statements rather that relative ones. Are exempt of this rule the modules relative to a submodule of OpenFisca, in order to improve the delimitation of internal and external interfaces.
+
+3. Always follow this order for your imports: system modules, third party modules, third party OpenFisca modules, external OpenFisca Core modules, internal OpenFisca Core modules.
+
+For example given:
+
+```
+/openfisca_core/axes/__init__.py
+/openfisca_core/axes/nothing.py
+/openfisca_core/axes/something.py
+```
+
+Whenever possible we should expect:
+
+```python
+# /openfisca_core/axes/nothing.py
+#
+# Yes
+
+import copy
+from typing import List
+
+import numpy
+from numpy.typing import ArrayLike
+
+from openfisca_country_template import entities
+
+from openfisca_core import tools
+from openfisca_core.variables import Variable
+
+from . import Something
+
+def do(this: List) -> ArrayLike:
+    that = copy.deepcopy(this)
+    array = numpy.ndarray(that)
+    return Something(entities.Person, Variable)
+```
+
+And avoid:
+
+```python
+# /openfisca_core/axes/nothing.py
+#
+# No
+
+from openfisca_country_template.entities import Person
+from openfisca_core import variables
+from openfisca_core.tools import assert_near
+from openfisca_core import axes
+
+from numpy import ndarray
+from copy import deepcopy
+import typing
+import numpy.typing
+
+def do(this: typing.List) -> numpy.typing.ArrayLike:
+    that = deepcopy(this)
+    array = ndarray(that)
+    return axes.Something(Person, variables.Variable)
+```