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Get and set column names #21
Description
Regarding column names, the next proposal, similar to what pandas currently does, uses a columns property to set and get columns names.
In #7, the preference is to restrict column names to string, and not allow duplicates.
The proposed API with an example is:
>>> df = dataframe({'col1': [1, 2], 'col2': [3, 4]})
>>> df.columns = 'foo', 'bar'
>>> df.columns = ['foo', 'bar']
>>> df.columns = map(str.upper, df.columns)
>>> df.columns
['FOO', 'BAR']And the next cases would fail:
>>> df.columns = 1
TypeError: Columns must be an iterable, not int
>>> df.columns = 'foo'
TypeError: Columns must be an iterable, not str
>>> df.columns = 'foo', 1
TypeError: Column names must be str, int found
>>> df.columns = 'foo', 'bar', 'foobar'
ValueError: Expected 2 column names, found 3
>>> df.columns = 'foo', 'foo'
ValueError: Column names cannot be duplicated. Found duplicates: fooSome things that people may want to discuss:
- Using a different name for the property (e.g.
column_names) - Being able to set a single column
df.columns[0] = 'foo'(the proposal don't allow it) - The return type of the columns (the proposal returns a Python list, pandas returns an Index)
- Setting the column of a dataframe with one column with
df.columns = 'foo'(the proposal requires an iterable, sodf.columns = ['foo']or equivalent is needed).
In case it's useful, this is the implementation of the examples:
import collections
import typing
class dataframe:
def __init__(self, data):
self._columns = list(data)
@property
def columns(self) -> typing.List[str]:
return self._columns
@columns.setter
def columns(self, names: typing.Iterable[str]):
if not isinstance(names, collections.abc.Iterable) or isinstance(names, str):
raise TypeError(f'Columns must be an iterable, not {type(names).__name__}')
names = list(names)
for name in names:
if not isinstance(name, str):
raise TypeError(f'Column names must be str, {type(name).__name__} found')
if len(names) != len(self._columns):
raise ValueError(f'Expected {len(self._columns)} column names, found {len(names)}')
if len(set(names)) != len(self._columns):
duplicates = set(name for name in names if names.count(name) > 1)
raise ValueError(f'Column names cannot be duplicated. Found duplicates: {", ".join(duplicates)}')
self._columns = names
Activity
datapythonista commentedon Jul 2, 2020
As pointed out in the meeting, the API in the description assumes the dataframe can be mutated in place (in this case the labels). This is something that has been discussed in #10, but it's still not decided which kind of API we want in terms of mutability. Those would be the main options:
rgommers commentedon Jun 25, 2021
The consensus ended up being: "no mutability". So we cannot set column names, and the interchange protocol has a simple
column_namesproperty at the dataframe level.This seems resolved, closing.