# API Reference¶

## Contact maps¶

 ContactCount(counter, object_f, n_x, n_y) Return object when dealing with contacts (residue or atom). ContactMap(frame[, query, haystack, cutoff, …]) Contact map (atomic and residue) for a single frame. ContactFrequency(trajectory[, query, …]) Contact frequency (atomic and residue) for a trajectory. ContactDifference(positive, negative) Contact map comparison (atomic and residue).

## Parallelization of ContactFrequency¶

 frequency_task Task-based implementation of ContactFrequency. DaskContactFrequency(client, filename[, …]) Dask-based parallelization of contact frequency.

## API naming conventions¶

There are several terms that are used throughout the API which are not completely standard. Understanding them, and how we use them, will make it much easier to understand the code.

Note

This section does not discuss the code style conventions we use, only the choice of specific words to mean specific things outside the normal scientific usage. For the code style, see the (to-be-written) developer documentation (or just use PEP8).

### Query/Haystack¶

Many functions in the API take the lists query and haystack as input. This nomenclature follows usage in MDTraj. These are lists of atom indices used in the contact search. Every pair will include one atom from query and one atom from haystack. In principle, the two lists are interchangeable. However, there are cases where the implementation will be faster if the query is the smaller of the two lists.

### Index/idx¶

Most of our return values are in terms of MDTraj Atom and Residue objects. This is because these are more readable, and provide the user with immediate access to useful context. However, there are times that what we really want is the atom or residue index number. For this, we include the idx suffix (e.g., most_common_atoms_idx). Note that these indices start from 0; this can be confusing when comparing to PDB entries where indexing is from 1.

### Most common¶

Several methods begin with most_common. The behavior for this is inspired by the behavior of collections.Counter.most_common(), which returns elements and there counts ordered from most to least. Note that, unlike the original, we usually do not implement a way to only return the first n results (although this may be added later).