It's turtles all the way down…

## Project description

Why yes, it’s turtles all the way down 🐢🐢🐢🐢…

This is an attempt at a cascading configuration library that I’d like to use. The basic idea is that you get a configuration object and read its properties for the values of options you’ve potentially defined all over the place, haha.

Behind the scenes TurtleConfig will look at command-line parameters, the environment, user configuration files, host/site configuration, and the app’s own defaults to find the best value. Typically in that order, although you’re welcome to modify that list of configuration sources to whatever you see fit.

Instead of the typical hodge-podge of custom code needed for that, setting up “TC” looks a little something like this. First, you’ll define a schema via a standard Python object. Either imported from a module, or locally via a class/object, which is easier to view here:

import os
from tconf import TurtleConfig

import schema as AppSchema  # or…

class AppSchema:
an_option = True  # simple options

class main:  # or perhaps a hierarchy
jpeg_quality = 95
file_path = '/foo'

Schema objects support type annotations and validation as well, but don’t need to be given most of the time.

Next, create the TurtleConfig object:

cfg = TurtleConfig(
'AppyMcApp',    # The app's name
sources = (     # A sequence of option sources
os.environ,
'{user_config_dir}/options.ini',  # appdirs
'{site_config_dir}/options.ini',  # appdirs
'/path/to/options.xml',  # or JSON, YAML, etc.
AppSchema,  # The schema/defaults object
),
ensure_paths=True,  # creates folders/files when needed
)

Virtual Paths

Note, the user_config_dir & site_config_dir variables above are provided by the appdirs module so you don’t have to worry about cross platform locations—unless you want to.

Tip: You may want to start logging at DEBUG level before loading TC, as it helps to visualize what is going on inside its shell. The out module is perhaps the easiest way to do that:

import sys, out
out.configure(level='debug' if '-d' in sys.argv else 'info')

## Try It!

First, install the small pure-python package, and off you go:

⏵ python3 -m pip install tconf

### Interfaces

Once loaded, there are two interfaces to get at options, via attributes or dictionary-like access:

# Attribute interface
>>> cfg.main.jpeg_quality
95

# Dictionary interface
>>> cfg['main.jpeg_quality']
95

Now, why would you use one form over the other?

Well, the first (attribute interface) is easier to type, read, and the design I originally wanted. However, it has limitations in a number of circumstances that are damn near impossible to overcome, which you’ll read about below.

So the second (dictionary form) is generally preferred unless the app has simple needs, such as a single-level configuration. An editor “snippet” can mitigate the extra keystrokes.

### Value Types

Some configuration sources are limited (in a good way)† in that they return option values only as strings. For example, the environment, strict-yaml, and .ini files have string-only values. However, our app is likely to need real types, such as integers, booleans, or lists of strings, etc. What to do?

Under TC, the types of option values are required to be the same type as their schema defaults, as defined in Python. Remember, defaults are found from the last class/module/object passed as a source, the AppSchema object as seen in the example above.

The default value types may be annotated, otherwise the given default will be inspected for its type as a fallback. This means you can skip having to write type annotations much of the time as they are inferred.

Points of interest:

• Turtle will attempt to convert or “coerce” string values gathered from a config source into the expected (annotated or inferred) non-string type.

• Values are then type checked via the typeguard module.

• Currently, simple and compound Python types are supported:

• str, int, float, bool

• list, tuple, dict, set

• List, Tuple, Sequence, Dict, Set (from le typing module)

• Union, Any, Optional, etc. (ditto)

• Annotations are required when complex, compound value types are needed, as defined with the stdlib typing module.

• Annotations may also support kwargs for an ArgumentParser, see below.

† Conversion of types is better done in the application-layer than in the file format to avoid unexpected edge-case bugs like “the Norway problem.”

## Configuration Sources

Each configuration source has an Adapter class to integrate the different interfaces into one. As mentioned, when looking for options, the sources are searched in order from top to bottom, front to back, until a suitable value is found. If an option is not found in any source, an AttributeError or KeyError (depending on interface) is raised to ensure bugs are found early.

### Environment Variables

Perhaps you’d like to override options with environment variables. This is what it looks like:

>>> os.environ['PY_APPYMCAPP.MAIN.JPEG_QUALITY'] = '94'
>>> cfg['main.jpeg_quality']
94

An environment variable matching one of our configuration values is uppercase and prefixed with PY_ and the application name. Both parts of the prefix are able to be modified by modifying the app_name and/or passing an env_prefix='…' to the TurtleConfig constructor.

Limitations:

Due to limits with how the environment adapter works, it cannot provide hierarchical access to settings via the attribute interface.

The reason is that the attributes are evaluated left to right. At access time, the object doesn’t yet have enough information to know if it should return the final value or continue down the attribute chain. It could decide on one or the other, leading to a number of broken cases from either decision. Bare attributes do work with the environment when options are kept to a single-level, however. As mentioned previously, dictionary-style access (shown above) works consistently.

### ConfigParser

.ini files have two levels by design and are great for config files. Therefore they do work hierarchically by default and would typically require exactly two levels.

There is one exception for convenience, however. If a single-level option is requested, the section [main] (configurable also) is tried as a fallback. This is so one can use a single-level as well as a dual-level config with ConfigParser, simply by putting root options under [main]:

⏵ cat test.ini
[main]
jpeg_quality = 96
# snip
>>> cfg.main.jpeg_quality
96
>>> cfg['main.jpeg_quality']
96
>>> cfg.jpeg_quality  # looks in [main] also
96

### JSON

JSON is not a great format for humans to edit, but still relatively common as configuration:

⏵ cat test.json
{   "an_option": true,
"main": {
"jpeg_quality": 96,
# ~snip~
>>> cfg.main.jpeg_quality
96
>>> cfg['main.jpeg_quality']
96
>>> cfg.does_not_exist
# …

>>> cfg.an_option
True

Compound data types are better encoded in the JSON itself rather than trying to smash “PyON” into strings.

### XML

Requires xmltodict:

⏵ pip3 install tconf[xml]  # or
⏵ pip3 install xmltodict

⏵ cat test.xml
<?xml version="1.0" encoding="UTF-8"?>
<root>
<an_option>true</an_option>
<a_null2/>
<main>
<jpeg_quality>96</jpeg_quality>
# ~snip~
>>> cfg.main.jpeg_quality
96
>>> cfg['main.jpeg_quality']
96
>>> cfg.a_null2
# implied None
>>> cfg.an_option
True

This Adapter is kinda weak so far, could use a rewrite.

Limitations:

• Throws out the root element for parity with other source types.

• Finds only the first node (tag) at each level due to a dictionary-like implementation.

• XML attributes are not currently reachable.  :-/

### Strict YAML

A much safer, simpler subset of YAML, which requires the strictyaml module:

⏵ pip3 install tconf[yaml]  # or
⏵ pip3 install strictyaml

⏵ cat test.yaml
an_option: true
a_null: null

main:
jpeg_quality: 96
# snip

See JSON above for similar Python snippet.

### Others

It’s trivial to add an adapter for other sources and file formats. First subclass adapters._Adapter and add an instance to the sources list. There is an file_adapter_map in the adapters module root to register file extensions to avoid having to pass an instance every time, if desired.

Tip: Additionally, passing adapters into the source list manually can also be used to give an Adapter different arguments than it would normally get.

See the next section for an example, and “use the source, Luke!”

### ArgumentParser

You may have been thinking, what about the command-line? Good news, there’s an ArgumentParser subclass available if you’d like all options presented auto-magically. Types and parameters are passed to ArgumentParser through annotations of the AppSchema object:

# appy.py
from tconf import TurtleConfig, TurtleArgumentParser # 👀

class AppSchema:
# snip…
class main:
# how to add a type via annotation,
# simple types are already detected however:
jpeg_quality: int = 95

# Also use annotations to pass a dictionary of
# kwargs to ArgParser, w/o descriptive help text:
jpeg_quality: dict(  # 👀
type=int,
desc='The jpeg quality level',
) = 95

tcfg = TurtleConfig(
'AppyMcApp',
sources = (
TurtleArgumentParser(AppSchema),  # 👀
# environment, config files, etc…
AppSchema,
),
)

Next, give it a try:

⏵ appy.py -h

usage: appy.py [-h] [--a-simple-option] [--main-jpeg-quality I]
[--main-sync-dates-to-filesystem] [--main-work-in-place]
[--rotate-resample S] [--sort-template S] [--specific-name S]

optional arguments:
-h, --help            show this help message and exit
--a-simple-option     🐢 (False, sets True)
--main-jpeg-quality I 🐢 The jpeg quality level (int)
--main-sync-dates-to… 🐢 (True, sets False)
--main-work-in-place  🐢 (False, sets True)
--rotate-resample S   🐢 (str)
--sort-template S     🐢 (str)
--specific-name S     🐢 (str)

Option help text can be set directly in the annotation with help='…'. To use a template for help based on a given description and auto-detected type, use desc='…' instead. The template is configurable as well via TurtleArgumentParser kwargs.

Hiding Options:

Options shown by an ArgumentParser can be hidden by passing the help=argparse.SUPPRESS value via the kwargs annotation to the option.

Given enough options, eventually the display of every possible option is too much, and suppression gets tedious. When something simpler to be presented to the end user is preferred, this also works as you’d expect:

from argparse import ArgumentParser

parser = ArgumentParser()
'--quality', default=cfg['main.jpeg_quality'],
)

Then, use args instead of cfg afterward for the options that take precedence. Remember—dots in an options string are presented as underscores in the ArgumentParser Namespace, and dashes on the command-line:

print('quality:', args.main_jpeg_quality)

## Misc

### Performance

“Why yes, it’s a racing Turtle.”

The TurtleConfig object caches results so it doesn’t have to go crawling through multiple files to find the value every time. So don’t get fancy with changing the environment on the fly, or editing config files unless you’ve cleared the cache with:

cfg.clear_turtle_cache()

### Exceptions

These are thrown when a error occurs.

• AttributeError,  attribute interface

• KeyError,  dict interface

Option value errors, when the value returned is bogus:

• ValueError,  wrong value in this context

• SyntaxError,  string unable to be evaluated

• TypeError,  wrong type returned

Ob-la-di ob-la-da life goes on bra…

### To Do:

Candidates for implementation:

• TOML

• .env files

• Windows registry

Released under the LGPL, version 3+.

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