[Numpy-discussion] RecArray.tolist() suggestion

[Francesc Alted]

A Dijous 15 Juliol 2004 17:21, Colin J. Williams va escriure:
> >What I propose is to be able to say:
> >>>>r["c1"][1]
> I would suggest going a step beyond this, so that one can have r.c1[1], 
> see the script below.

Yeah. I've implemented something similar to access column elements for
pytables Table objects. However, the problem in this case is that there are
already attributes that "pollute" the column namespace, so that a column
named "size" collides with the size() method.

I came up with a solution by adding a new "cols" attribute to the Table
object that is an instance of a simple class named Cols with no attributes
that can pollute the namespace (except some starting by "__" or "_v_").
Then, it is just a matter of provide functionality to access the different
columns. In that case, when a reference of a column is made, another object
(instance of Column class) is returned. This Column object is basically an
accessor to column values with a __getitem__() and __setitem__() methods.
That might sound complicated, but it is not. I'm attaching part of the
relevant code below.

I personally like that solution in the context of pytables because it
extends the "natural naming" convention quite naturally. A similar approach
could be applied to RecArray objects as well, although numarray might (and
probably do) have other usage conventions.

> I have not explored the assignment of a value to r.c1.[1], but it seems 
> to be achievable.

in the schema I've just proposed the next should be feasible:

value = r.cols.c1[1]
r.cols.c1[1] = value


-- 
Francesc Alted


-----------------------------------------------------------------
class Cols(object):
    """This is a container for columns in a table

    It provides methods to get Column objects that gives access to the
    data in the column.

    Like with Group instances and AttributeSet instances, the natural
    naming is used, i.e. you can access the columns on a table like if
    they were normal Cols attributes.
    
    Instance variables:

        _v_table -- The parent table instance
        _v_colnames -- List with all column names

    Methods:
    
        __getitem__(colname)
        
    """

    def __init__(self, table):
        """Create the container to keep the column information.

        table -- The parent table
        
        """
        self.__dict__["_v_table"] = table
        self.__dict__["_v_colnames"] = table.colnames
        # Put the column in the local dictionary
        for name in table.colnames:
            self.__dict__[name] = Column(table, name)

    def __len__(self):
        return self._v_table.nrows

    def __getitem__(self, name):
        """Get the column named "name" as an item."""

        if not isinstance(name, types.StringType):
            raise TypeError, \
"Only strings are allowed as keys of a Cols instance. You passed object: %s" % name
        # If attribute does not exist, return None
        if not name in self._v_colnames:
            raise AttributeError, \
"Column name '%s' does not exist in table:\n'%s'" % (name, str(self._v_table))

        return self.__dict__[name]

    def __str__(self):
        """The string representation for this object."""
        # The pathname
        pathname = self._v_table._v_pathname
        # Get this class name
        classname = self.__class__.__name__
        # The number of columns
        ncols = len(self._v_colnames)
        return "%s.cols (%s), %s columns" % (pathname, classname, ncols)

    def __repr__(self):
        """A detailed string representation for this object."""

        out = str(self) + "\n"
        for name in self._v_colnames:
            # Get this class name
            classname = getattr(self, name).__class__.__name__
            # The shape for this column
            shape = self._v_table.colshapes[name]
            # The type
            tcol = self._v_table.coltypes[name]
            if shape == 1:
                shape = (1,)
            out += "  %s (%s%s, %s)" % (name, classname, shape, tcol) + "\n"
        return out

               
class Column(object):
    """This is an accessor for the actual data in a table column

    Instance variables:

        table -- The parent table instance
        name -- The name of the associated column

    Methods:
    
        __getitem__(key)
        
    """

    def __init__(self, table, name):
        """Create the container to keep the column information.

        table -- The parent table instance
        name -- The name of the column that is associated with this object
        
        """
        self.table = table
        self.name = name
        # Check whether an index exists or not
        iname = "_i_"+table.name+"_"+name
        self.index = None
        if iname in table._v_parent._v_indices:
            self.index = Index(where=self, name=iname,
                               expectedrows=table._v_expectedrows)
        else:
            self.index = None

    def __getitem__(self, key):
        """Returns a column element or slice

        It takes different actions depending on the type of the
        "key" parameter:

        If "key" is an integer, the corresponding element in the
        column is returned as a NumArray/CharArray, or a scalar
        object, depending on its shape. If "key" is a slice, the row
        slice determined by this slice is returned as a NumArray or
        CharArray object (whatever is appropriate).

        """
        
        if isinstance(key, types.IntType):
            if key < 0:
                # To support negative values
                key += self.table.nrows
            (start, stop, step) = processRange(self.table.nrows, key, key+1, 1)
            return self.table._read(start, stop, step, self.name, None)[0]
        elif isinstance(key, types.SliceType):
            (start, stop, step) = processRange(self.table.nrows, key.start,
                                               key.stop, key.step)
            return self.table._read(start, stop, step, self.name, None)
        else:
            raise TypeError, "'%s' key type is not valid in this context" % \
                  (key)

    def __str__(self):
        """The string representation for this object."""
        # The pathname
        pathname = self.table._v_pathname
        # Get this class name
        classname = self.__class__.__name__
        # The shape for this column
        shape = self.table.colshapes[self.name]
        if shape == 1:
            shape = (1,)
        # The type
        tcol = self.table.coltypes[self.name]
        return "%s.cols.%s (%s%s, %s)" % (pathname, self.name,
                                          classname, shape, tcol)

    def __repr__(self):
        """A detailed string representation for this object."""
        return str(self)