Pandas: Indexes#

import pandas as pd

persons = pd.DataFrame({
    'firstname': ['Joerg',                  'Johanna',           'Caro',              'Philipp'          ],
    'lastname':  ['Faschingbauer',          'Faschingbauer',     'Faschingbauer',     'Lichtenberger'    ],
    'email':     ['office@faschingbauer.co.at', 'johanna@email.com', 'caro@email.com',    'philipp@email.com'],
    'age':       [56,                       27,                  25,                  37                 ],
})

Default Index: Row Number#

persons
firstname lastname email age
0 Joerg Faschingbauer office@faschingbauer.co.at 56
1 Johanna Faschingbauer johanna@email.com 27
2 Caro Faschingbauer caro@email.com 25
3 Philipp Lichtenberger philipp@email.com 37

  • See how rows are numbered

  • No column name given

  • ⟶ default index

persons.index

RangeIndex(start=0, stop=4, step=1)

Setting Custom Index#

  • Notice how email appears to be unique

  • ⟶ could be used as an index

    persons.set_index('email')
    firstname lastname age
    email
    office@faschingbauer.co.at Joerg Faschingbauer 56
    johanna@email.com Johanna Faschingbauer 27
    caro@email.com Caro Faschingbauer 25
    philipp@email.com Philipp Lichtenberger 37

  • This does not change anything

  • Returns modified copy (could be assigned to another variable that you continue to work with, for example)

  • persons is still the same as before

    persons
    firstname lastname email age
    0 Joerg Faschingbauer office@faschingbauer.co.at 56
    1 Johanna Faschingbauer johanna@email.com 27
    2 Caro Faschingbauer caro@email.com 25
    3 Philipp Lichtenberger philipp@email.com 37

Setting Custom Index, inplace=True#

  • Many (but not all) DataFrame methods support an inplace parameter

  • Default False

    • ⟶ no change

    • Returns a modified copy of the DataFrame object

  • Nice for trying around on a large dataset that we don’t want to damage

  • Add inplace if everything works

  • ⟶ No return value

    persons.set_index('email', inplace=True)

  • Modified object in-place

    persons
    firstname lastname age
    email
    office@faschingbauer.co.at Joerg Faschingbauer 56
    johanna@email.com Johanna Faschingbauer 27
    caro@email.com Caro Faschingbauer 25
    philipp@email.com Philipp Lichtenberger 37

  • Index has changed

    persons.index

    Index(['office@faschingbauer.co.at', 'johanna@email.com', 'caro@email.com',
       'philipp@email.com'],
      dtype='str', name='email')

Custom Index, And loc[]#

  • loc[] selects by row label (⟶ index)

  • Row labels are not row numbers anymore ⟶ cannot be used as row labels

    persons.loc[0]

    ---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
File ~/My-Environments/jfasch-home/lib64/python3.14/site-packages/pandas/core/indexes/base.py:3641, in Index.get_loc(self, key)
   3640 try:
-> 3641     return self._engine.get_loc(casted_key)
   3642 except KeyError as err:

File pandas/_libs/index.pyx:168, in pandas._libs.index.IndexEngine.get_loc()
--> 168 'Could not get source, probably due dynamically evaluated source code.'

File pandas/_libs/index.pyx:176, in pandas._libs.index.IndexEngine.get_loc()
--> 176 'Could not get source, probably due dynamically evaluated source code.'

File pandas/_libs/index.pyx:583, in pandas._libs.index.StringObjectEngine._check_type()
--> 583 'Could not get source, probably due dynamically evaluated source code.'

KeyError: 0

The above exception was the direct cause of the following exception:

KeyError                                  Traceback (most recent call last)
Cell In[9], line 1
----> 1 persons.loc[0]

File ~/My-Environments/jfasch-home/lib64/python3.14/site-packages/pandas/core/indexing.py:1207, in _LocationIndexer.__getitem__(self, key)
   1205 maybe_callable = com.apply_if_callable(key, self.obj)
   1206 maybe_callable = self._raise_callable_usage(key, maybe_callable)
-> 1207 return self._getitem_axis(maybe_callable, axis=axis)

File ~/My-Environments/jfasch-home/lib64/python3.14/site-packages/pandas/core/indexing.py:1449, in _LocIndexer._getitem_axis(self, key, axis)
   1447 # fall thru to straight lookup
   1448 self._validate_key(key, axis)
-> 1449 return self._get_label(key, axis=axis)

File ~/My-Environments/jfasch-home/lib64/python3.14/site-packages/pandas/core/indexing.py:1399, in _LocIndexer._get_label(self, label, axis)
   1397 def _get_label(self, label, axis: AxisInt):
   1398     # GH#5567 this will fail if the label is not present in the axis.
-> 1399     return self.obj.xs(label, axis=axis)

File ~/My-Environments/jfasch-home/lib64/python3.14/site-packages/pandas/core/generic.py:4253, in NDFrame.xs(self, key, axis, level, drop_level)
   4249                     new_index = index[loc : loc + 1]
   4250                 else:
   4251                     new_index = index[loc]
   4252         else:
-> 4253             loc = index.get_loc(key)
   4254 
   4255             if isinstance(loc, np.ndarray):
   4256                 if loc.dtype == np.bool_:

File ~/My-Environments/jfasch-home/lib64/python3.14/site-packages/pandas/core/indexes/base.py:3648, in Index.get_loc(self, key)
   3643     if isinstance(casted_key, slice) or (
   3644         isinstance(casted_key, abc.Iterable)
   3645         and any(isinstance(x, slice) for x in casted_key)
   3646     ):
   3647         raise InvalidIndexError(key) from err
-> 3648     raise KeyError(key) from err
   3649 except TypeError:
   3650     # If we have a listlike key, _check_indexing_error will raise
   3651     #  InvalidIndexError. Otherwise we fall through and re-raise
   3652     #  the TypeError.
   3653     self._check_indexing_error(key)

KeyError: 0

  • New row label: email

    persons.loc['office@faschingbauer.co.at']

    firstname            Joerg
lastname     Faschingbauer
age                     56
Name: office@faschingbauer.co.at, dtype: object

    persons.loc[['office@faschingbauer.co.at', 'johanna@email.com']]
    firstname lastname age
    email
    office@faschingbauer.co.at Joerg Faschingbauer 56
    johanna@email.com Johanna Faschingbauer 27

Custom Index, And iloc[]#

  • iloc[] selects by row number

  • ⟶ still valid as before

persons.iloc[0]

firstname            Joerg
lastname     Faschingbauer
age                     56
Name: office@faschingbauer.co.at, dtype: object

persons.iloc[[0, 1]]
firstname lastname age
email
office@faschingbauer.co.at Joerg Faschingbauer 56
johanna@email.com Johanna Faschingbauer 27

Sorting DataFrame Object By Index Column#

  • DataFrame.sort_index(): non inplace by default ⟶ returns modified copy

    persons.sort_index(ascending=True)
    firstname lastname age
    email
    caro@email.com Caro Faschingbauer 25
    johanna@email.com Johanna Faschingbauer 27
    office@faschingbauer.co.at Joerg Faschingbauer 56
    philipp@email.com Philipp Lichtenberger 37

  • Sorting in place

    persons.sort_index(ascending=True, inplace=True)

    persons
    firstname lastname age
    email
    caro@email.com Caro Faschingbauer 25
    johanna@email.com Johanna Faschingbauer 27
    office@faschingbauer.co.at Joerg Faschingbauer 56
    philipp@email.com Philipp Lichtenberger 37