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from collections import OrderedDict
from copy import copy
from agate.rows import Row
def compute(self, computations, replace=False):
"""
Create a new table by applying one or more :class:`.Computation` instances
to each row.
:param computations:
A sequence of pairs of new column names and :class:`.Computation`
instances.
:param replace:
If :code:`True` then new column names can match existing names, and
those columns will be replaced with the computed data.
:returns:
A new :class:`.Table`.
"""
column_names = list(copy(self._column_names))
column_types = list(copy(self._column_types))
for new_column_name, computation in computations:
new_column_type = computation.get_computed_data_type(self)
if new_column_name in column_names:
if not replace:
raise ValueError(
'New column name "%s" already exists. Specify replace=True to replace with computed data.'
)
i = column_names.index(new_column_name)
column_types[i] = new_column_type
else:
column_names.append(new_column_name)
column_types.append(new_column_type)
computation.validate(self)
new_columns = OrderedDict()
for new_column_name, computation in computations:
new_columns[new_column_name] = computation.run(self)
new_rows = []
for i, row in enumerate(self._rows):
# Slow version if using replace
if replace:
values = []
for j, column_name in enumerate(column_names):
if column_name in new_columns:
values.append(new_columns[column_name][i])
else:
values.append(row[j])
# Faster version if not using replace
else:
values = row.values() + tuple(c[i] for c in new_columns.values())
new_rows.append(Row(values, column_names))
return self._fork(new_rows, column_names, column_types)
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