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Droop is a Python-based package for counting STV elections.

The primary motivation for Droop is to provide a framework for implementing STV counters in a manner that facilitates verification and certification. Rule implementations are self-contained and independent of other rules. Rules can, in general, be expressed in terms of their statutory language. Extensive and extensible unit tests provide additional validation support.

Droop's features include:

• STV rule implementations are self-contained, for clarity and straightforward validation
• Droop's framework provides housekeeping support tailored for STV implementation, keeping the rule implementations uncluttered.
• The framework includes interchangeable arithmetic support for integer, decimal fixed-point, guarded decimal fixed-point (quasi-exact) and rational (exact) arithmetic.
• Droop includes implementations of several STV rules, including reference rules from the Proportional Representation Foundation and an example implementation of Minneapolis MN's STV rule; more are planned.
• Parametric implementations of Meek, Warren and Weighted Inclusive Gregory (WIGM) methods are available, so that ad hoc rules can be easily assembled and compared from the user interface.
• New rule implementations can be "dropped in" without any changes to the framework.
• The framework, rule implementations and user interface are decoupled; not only are new rule implementations easy to add, but so are new user interfaces. The current implementation includes a general-purpose CLI as well as examples of specialized CLI interfaces for specific rules. A web-based UI is planned but not yet implemented.
• The framework and rule implementations are readily embedded as an STV counting engine in third-party software.
• All IO treated as UTF-8.

See Droop Architecture in the wiki for an overview of Droop's structure.

Currently (v0.14), the unit tests run correctly on macOS 10.15.4 with Python 3.7.3.