class _Columns:
"""_Columns(columns) -> _Columns"""
__slots__ = slots("column_index column_names")
def __init__(self, columns):
"""Initializes Columns instance with names and data types."""
self.__column_index = 1
self.__column_names = UniqueDict()
for name, data_type in columns:
self.add(name, data_type)
def __contains__(self, name):
"""Checks if the named column already exists."""
return name in self.__column_names
def __len__(self):
"""Returns the number of columns recognizes."""
return len(self.__column_names)
def __iter__(self):
"""Iterates over columns in sorted order."""
cache = []
for name, (data_type, index) in self.__column_names.items():
cache.append((index, name, data_type))
for item in sorted(cache):
yield item
def __getitem__(self, name):
"""Returns requested information on the given column name."""
return self.__column_names[name]
def __getstate__(self):
"""Provides support for class instances to be pickled."""
return self.__column_index, self.__column_names
def __setstate__(self, state):
"""Sets the state while object in being unpickled."""
self.__column_index, self.__column_names = state
def copy(self):
"""Creates a copy of the known columns."""
copy = type(self)([])
copy.__column_index = self.__column_index
copy.__column_names = self.__column_names.copy()
return copy
def add(self, name, data_type):
"""Adds a column name with data type and assigns an index."""
index = self.__column_index
self.__column_names[name] = data_type, index
self.__column_index += 1
return index
def drop(self, name):
"""Removes all information regarding the named column."""
index = self.__column_names[name][1]
del self.__column_names[name]
return index
def alter(self, name, data_type):
"""Changes the data type of the named column."""
index = self.__column_names[name][1]
self.__column_names.replace(name, (data_type, index))
return index
def rename(self, old, new):
"""Renames a column from old name to new name."""
self.__column_names[new] = self.__column_names[old]
del self.__column_names[old]
