conda config and context

The context object is central to many parts of the conda codebase. It serves as a centralized repository of settings. You normally import the singleton and access its (many) attributes directly:

from conda.base.context import context

# False

This singleton is initialized from a cascade of different possible sources. From lower to higher precedence:

  1. Default values hardcoded in the Context class. These are defined via class attributes.

  2. Values defined in the configuration files (.condarc), which have their own precedence.

  3. Values set by the corresponding command line arguments, if any.

  4. Values defined by their corresponding CONDA_* environment variables, if present.

The mechanism implementing this behavior is an elaborate object with several types of objects involved.

Anatomy of the Context class

conda.base.context.Context is an conda-specific subclass of the application-agnostic conda.common.configuration.Configuration class. This class implements the precedence order for the instantiation of each defined attribute, as well as the overall validation logic and help message reporting. But that’s it, it’s merely a storage of ParameterLoader objects which, in turn, instantiate the relevant Parameter subclasses in each attribute. Roughly:

class MyConfiguration(Configuration):
    string_field = ParameterLoader(PrimitiveParameter("default", str))
    list_of_int_field = ParameterLoader(SequenceParameter([1, 2, 3], int))
    map_of_foat_values_field = ParameterLoader(MapParameter({"key": 1.0}, float))

When MyConfiguration is instantiated, those class attributes are populated by the .raw_data dicionary that has been filled in with the values coming from the precedence chain stated above. The raw_data dictionary contains RawParameter objects, subclassed to deal with the specifics of their origin (YAML file, environment variable, command line flag). Each ParameterLoader object will pass the RawParameter object to the .load() method of its relevant Parameter subclass, which are designed to return their corresponding LoadedParameter object counterpart.

It’s a bit confusing, but the delegation happens like this:

  1. The Configuration subclass parses the raw values of the possible origins and stores them as the relevant RawParameter objects, which can be:

    • EnvRawParameter: for those coming from an environment variable

    • ArgParseRawParameter: for those coming from a command line flag

    • YamlRawParameter: for those coming from a configuration file

    • DefaultValueRawParameter: for those coming from the default value given to ParameterLoader

  2. Each Configuration attribute is a ParameterLoader, which implements the property protocol via __get__. This means that, upon attribute access (e.g. MyConfiguration.string_field), the ParameterLoader can execute the loading logic. This means finding potential type matches in the raw data, loading them as LoadedParameter objects and merging them with the adequate precedence order.

The merging policy depends on the (Loaded)Parameter subtype. Below is a list of available subtypes:

  • PrimitiveParameter: holds a single scalar value of type str, int, float, complex, bool or NoneType.

  • SequenceParameter: holds an iterable (list) of other Parameter objects.

  • MapParameter: holds a mapping (dict) of other Parameter objects.

  • ObjectParameter: holds an object with attributes set to Parameter objects.

The main goal of the Parameter objects is to implement how to typify and turn the raw values into their Loaded counterparts. These implement the validation routines and define how parameters for the same key should be merged:

  • PrimitiveLoadedParameter: value with highest precedence replaces the existing one.

  • SequenceLoadedParameter: extends with no duplication, keeping precedence.

  • MapLoadedParameter: cascading updates, highest precedence kept.

  • ObjectLoadedParameter: same as Map.

After all of this, the LoadedParameter objects are typified: this is when type validation is performed. If everything goes well, you obtain your values just fine. If not, the validation errors are raised.

Take into account that the result is cached for faster subsequent access. This means that even if you change the value of the environment variables responsible for a given setting, this won’t be reflected in the context object until you refresh it with conda.base.context.reset_context().

Do not modify the Context object!

ParameterLoader does not implement the __set__ method of the property protocol, so you can freely override an attribute defined in a Configuration subclass. You might think that this will redefine the value after passing through the validation machinery, but that’s not true. You will simply overwrite it entirely with the raw value and that’s probably not what you want.

Instead, consider the context object immutable. If you need to change a setting at runtime, it is probably A Bad Idea. The only situation where this is acceptable is during testing.

Setting values in the different origins

There’s some magic behind the possible origins for the settings values. How these are tied to the final Configuration object might not be obvious at first. This is different for each RawParameter subclass:

  • DefaultValueRawParameter: Users will never see this one. It only wraps the default value passed to the ParameterLoader class. Safe to ignore.

  • YamlRawParameter: This one takes a YAML file and parses it as a dictionary. The keys in this file must match the attribute names in the Configuration class exactly (or one of their aliases). Matching happens automatically once this is properly set up. How the values are parsed depends on the YAML Loader, set internally by conda.

  • EnvRawParameter: Values coming from certain environment variables can make it to the Configuration instance, provided they are formatted as <APP_NAME>_<PARAMETER_NAME>, all uppercase. The app name is defined by the Configuration subclass. The parameter name is defined by the attribute name in the class, transformed to upper case. For example, context.ignore_pinned can be set with CONDA_IGNORE_PINNED. The value of the variable is parsed in different ways depending on the type:

    • PrimitiveParameter is the easy one. The environment variable string is parsed as the expected type. Booleans are a bit different since several strings are recognized as such, and in a case-insensitive way:

      • True can be set with true, yes, on and y.

      • False can be set with false, off, n, no, non, none and "" (empty string).

    • SequenceParameter can specify their own delimiter (e.g. ,), so the environment variable string is processed into a list.

    • MapParameter and ObjectParameter do not support being set with environment variables.

  • ArgParseRawParameter: These are a bit different because there is no automated mechanism that ties a given command line flag to the context object. This means that if you add a new setting to the Context class and you want that available in the CLI as a command line flag, you have to add it yourself. If that’s the case, refer to conda.cli.conda_argparse and make sure that the dest value of your argparse.Argument matches the attribute name in Context. This way, Configuration.__init__ can take the argparse.Namespace object, turn it into a dictionary, and make it pass through the loading machinery.