struct.py

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#
# LSST Data Management System
# Copyright 2008, 2009, 2010, 2011 LSST Corporation.
#
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# LSST Project (http://www.lsst.org/).
#
# This program is free software: you can redistribute it and/or modify
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# GNU General Public License for more details.
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#

__all__ = ["Struct"]


class Struct:
    """A container to which you can add fields as attributes.

    Parameters
    ----------
    keyArgs
        keyword arguments specifying fields and their values.

    Notes
    -----
    Intended to be used for the return value from `~lsst.pipe.base.Task.run` and other `~lsst.pipe.base.Task`
    methods, and useful for any method that returns multiple values.

    The intent is to allow accessing returned items by name, instead of unpacking a tuple.
    This makes the code much more robust and easier to read. It allows one to change what values are returned
    without inducing mysterious failures: adding items is completely safe, and removing or renaming items
    causes errors that are caught quickly and reported in a way that is easy to understand.

    The primary reason for using Struct instead of dict is that the fields may be accessed as attributes,
    e.g. ``aStruct.foo`` instead of ``aDict["foo"]``. Admittedly this only saves a few characters, but it
    makes the code significantly more readable.

    Struct is preferred over named tuples, because named tuples can be used as ordinary tuples, thus losing
    all the safety advantages of Struct. In addition, named tuples are clumsy to define and Structs
    are much more mutable (e.g. one can trivially combine Structs and add additional fields).

    Examples
    --------
    >>> myStruct = Struct(
    >>>     strVal = 'the value of the field named "strVal"',
    >>>     intVal = 35,
    >>> )

    """

    def __init__(self, **keyArgs):
        for name, val in keyArgs.items():
            self.__safeAdd(name, val)

    def __safeAdd(self, name, val):
        """Add a field if it does not already exist and name does not start with ``__`` (two underscores).

        Parameters
        ----------
        name : `str`
            Name of field to add.
        val : object
            Value of field to add.

        Raises
        ------
        RuntimeError
            Raised if name already exists or starts with ``__`` (two underscores).
        """
        if hasattr(self, name):
            raise RuntimeError("Item %s already exists" % (name,))
        if name.startswith("__"):
            raise RuntimeError("Item name %r invalid; must not begin with __" % (name,))
        setattr(self, name, val)

    def getDict(self):
        """Get a dictionary of fields in this struct.

        Returns
        -------
        structDict : `dict`
            Dictionary with field names as keys and field values as values. The values are shallow copies.
        """
        return self.__dict__.copy()

    def mergeItems(self, struct, *nameList):
        """Copy specified fields from another struct, provided they don't already exist.

        Parameters
        ----------
        struct : `Struct`
            `Struct` from which to copy.
        *nameList : `str`
            All remaining arguments are names of items to copy.

        Raises
        ------
        RuntimeError
            Raised if any item in nameList already exists in self (but any items before the conflicting item
            in nameList will have been copied).

        Examples
        --------
        For example::

            foo.copyItems(other, "itemName1", "itemName2")

        copies ``other.itemName1`` and ``other.itemName2`` into self.
        """
        for name in nameList:
            self.__safeAdd(name, getattr(struct, name))

    def copy(self):
        """Make a one-level-deep copy (values are not copied).

        Returns
        -------
        copy : `Struct`
            One-level-deep copy of this Struct.
        """
        return Struct(**self.getDict())

    def __eq__(self, other):
        return self.__dict__ == other.__dict__

    def __len__(self):
        return len(self.__dict__)

    def __repr__(self):
        itemList = ["%s=%r" % (name, val) for name, val in self.getDict().items()]
        return "%s(%s)" % (self.__class__.__name__, "; ".join(itemList))