config2py.base

Base for getting configs from various sources and formats

class config2py.base.FuncBasedGettableContainer(getter: ~typing.Callable[[KT], VT], val_is_valid: ~typing.Callable[[VT], bool] = <function always_true>, config_not_found_exceptions: ~typing.Tuple[~typing.Type[Exception], ...] = (<class 'Exception'>,))[source]

A class that wraps a Callable[[KT], VT] function so it has a (partial) Mapping[KT, TT] interface. It is “partial” in the sense that it only implements __getitem__, raise a KeyError when a key can’t be computed. This is the standard for Mapping types, which enables us to use the FuncBasedGettable in a collections.ChainMap to catch the error and move on to the next source.

>>> def getter(k):
...     if k == 'foo':
...         return 'quux'
...     elif k == 'green':
...         return 'eggs'
...     else:
...         raise RuntimeError(f"I don't handle that: {k}")
>>> gc = FuncBasedGettableContainer(getter)
>>> gc['foo']
'quux'
>>> gc['green']
'eggs'

Observe below that though the getter function raises a RuntimeError, the FuncBasedGettableContainer raises a KeyError, to conform to the Mapping protocol.

>>> gc['no_a_key']  
Traceback (most recent call last):
...
KeyError: 'There was an exception ... : "I don't handle that: no_a_key"'

Note that by default, FuncBasedGettableContainer will catch all Exception exceptions, but you can specify a different set of exceptions to catch.

Note as well that you can specify a val_is_valid function that will be used to check the value returned by the getter function. If the value is not valid, a KeyError will also be raised. This is useful, for example, when you have a function that returns a sentinel like None instead of raising an exception, but you want to treat that as a KeyError.

>>> def getter(k):
...     if k == 'foo':
...         return 'quux'
...     elif k == 'green':
...         return 'eggs'
...     else:
...         return None
>>> gc = FuncBasedGettableContainer(getter, val_is_valid=lambda x: x is not None)
>>> gc['foo']
'quux'
>>> gc['no_a_key']
Traceback (most recent call last):
...
KeyError: 'Value for key no_a_key is not valid: None'

val_is_valid() → bool: Function that just returns True.

class config2py.base.GettableContainer(*args, **kwargs)[source]

Containers that are “gettable””.

By “gettable”, we mean that we can fetch an element from obj with brackets: obj[k]. That is, obj has a __getitem__ method. A Container means that obj has a __contains__ method, i.e. the expression k in obj is valid.

>>> isinstance(3, GettableContainer)  # 3 is not Gettable (can't do 3[...])
False

But dict, list, and str are GettableContainer:

>>> isinstance([1, 2, 3], GettableContainer)
True
>>> isinstance({'foo': 'bar'}, GettableContainer)
True
>>> isinstance('foo', GettableContainer)
True

Note that so are their types:

>>> all(isinstance(c, GettableContainer) for c in (list, dict, str))
True

But set is not a GettableContainer.

>>> myset = {1, 2, 3}
>>> isinstance(myset, GettableContainer)
False

This is because a set is a Container, but it is not gettable:

>>> 4 in myset  # set is a container
False
>>> myset[4]  # ... but not gettable
Traceback (most recent call last):
...
TypeError: 'set' object is not subscriptable

config2py.base.get_config(key: KT | None = None, sources: ~typing.Iterable[~config2py.base.GettableContainer | ~typing.Callable[[KT], VT]] | None = None, *, default: VT = Sentinel('no_default'), egress: ~typing.Callable[[KT, VT], VT] | None = None, val_is_valid: ~typing.Callable[[VT], bool] = <function always_true>, config_not_found_exceptions: ~typing.Tuple[~typing.Type[Exception], ...] = (<class 'Exception'>,))[source]

Get a config value from a list of sources

A source can be a function or a GettableContainer. (A GettableContainer is anything that can be indexed with brackets: obj[k], like dict, list, str, etc..).

Let’s take two sources: a dict and a Callable.

>>> def func(k):
...     if k == 'foo':
...         return 'quux'
...     elif k == 'green':
...         return 'eggs'
...     else:
...         raise RuntimeError(f"I don't handle that: {k}")
>>> dict_ = {'foo': 'bar', 'baz': 'qux'}
>>> sources = [func, dict_]

See that get_config go through the sources in the order they were listed, and returns the first value it finds (or manages to compute) for the key:

get_config finds 'foo' in the very first source (func):

>>> get_config('foo', sources)
'quux'

But baz makes func raise an error, so it goes to the next source: dict_. There, it finds 'baz' and returns its value:

>>> get_config('baz', sources)
'qux'

On the other hand, no one manages to find a config value for 'no_a_key', so get_config raises an error:

>>> get_config('no_a_key', sources)
Traceback (most recent call last):
...
config2py.errors.ConfigNotFound: Could not find config for key: no_a_key

But if you provide a default value, it will return that instead:

>>> get_config('no_a_key', sources, default='default')
'default'

You can also provide a function that will be called on the value before it is returned. This is useful if you want to do some post-processing on the value, or if you want to make sure that the value is of a certain type:

This “search the next source if the previous one fails” behavior may not be what you want in some situations, since you’d be hiding some errors that you might want to be aware of. This is why allow you to specify what exceptions should actually be considered as “config not found” exceptions, through the config_not_found_exceptions argument, which defaults to Exception.

Further, your sources may return a value, but not one that you consider valid: For example, a sentinel like None. In this case you may want the search to continue. This is what the val_is_valid argument is for. It is a function that takes a value and returns a boolean. If it returns False, the search will continue. If it returns True, the search will stop and the value will be returned.

Finally, we have egress : Callable[[KT, TT], VT]. This is a function that takes a key and a value, and returns a value. It is called after the value has been found, and its return value is the one that is returned by get_config. This is useful if you want to do some post-processing on the value, or before you return the value, or if you want to do some caching.

>>> config_store = dict()
>>> def store_before_returning(k, v):
...    config_store[k] = v
...    return v
>>> get_config('foo', sources, egress=store_before_returning)
'quux'
>>> config_store
{'foo': 'quux'}

Note that a source can be a callable or a GettableContainer (most of the time, a Mapping (e.g. dict)). Here, you should be compelled to use the resources of dol (https://pypi.org/project/dol/) which will allow you to make ``Mapping``s for all sorts of data sources.

For more info, see: https://github.com/i2mint/config2py/issues/4

config2py.base.gettable_containers(sources: ~typing.Iterable[~config2py.base.GettableContainer | ~typing.Callable[[KT], VT]], val_is_valid: ~typing.Callable[[VT], bool] = <function always_true>, config_not_found_exceptions: ~typing.Tuple[~typing.Type[Exception], ...] = (<class 'Exception'>,)) → Iterable[GettableContainer][source]: Convert an iterable of sources into GettableContainers

config2py.base.sources_chainmap(sources: ~typing.Iterable[~config2py.base.GettableContainer | ~typing.Callable[[KT], VT]], val_is_valid: ~typing.Callable[[VT], bool] = <function always_true>, config_not_found_exceptions: ~typing.Tuple[~typing.Type[Exception], ...] = (<class 'Exception'>,)) → ChainMap[source]: Create a ChainMap from a list of sources

config2py.base.user_gettable(save_to: ~typing.MutableMapping | ~typing.Callable[[KT, VT], ~typing.Any] | None = None, *, prompt_template='Enter a value for {}: ', egress: ~typing.Callable | None = None, user_asker=<function ask_user_for_input>, val_is_valid: ~typing.Callable[[VT], bool] = <function always_true>, config_not_found_exceptions: ~typing.Tuple[~typing.Type[Exception], ...] = (<class 'Exception'>,))[source]

Create a GettableContainer that asks the user for a value, optionally saving it.

Parameters:

save_to – A MutableMapping to save the user’s response to. If None, the user’s response is not saved.
prompt_template – A template string to prompt the user with. It should contain a placeholder for the key, e.g. "Enter a value for {}: ".
egress – A function to apply to the user’s response before returning it. This can be used to validate the response, for example.
user_asker – A function that asks the user for input. It should take a prompt string and return the user’s response.
val_is_valid – A function that takes a value and returns a boolean. If it returns False, the user will be asked for a new value.
config_not_found_exceptions – An iterable of exceptions that should be considered as “config not found” exceptions. If the user’s response raises one of these exceptions, the user will be asked for a new value.

Returns:

A GettableContainer that asks the user for a value, optionally saving it.

Example:

>>> s = user_gettable()
>>> v = s['SOME_KEY']  
'SOME_VAL'

This will trigger a prompt for the user to enter the value of SOME_KEY. When they do (say they entered ‘SOME_VAL’) it will return that value.

And if you specify a save_to store (usually a persistent MutableMapping made with the dol package) then it will save the value to that store for future use.

>>> d = dict(some='store')
>>> s = user_gettable(save_to=d)
>>> s['SOME_KEY']  
'SOME_VAL'
>>> d  
{'some': 'store', 'SOME_KEY': 'SOME_VAL'}