Pandas: Indexes#

import pandas as pd

persons = pd.DataFrame({
    'firstname': ['Joerg',                  'Johanna',           'Caro',              'Philipp'          ],
    'lastname':  ['Faschingbauer',          'Faschingbauer',     'Faschingbauer',     'Lichtenberger'    ],
    'email':     ['jf@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	jf@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
jf@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	jf@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
jf@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(['jf@faschingbauer.co.at', 'johanna@email.com', 'caro@email.com',
       'philipp@email.com'],
      dtype='object', 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/python3.13/site-packages/pandas/core/indexes/base.py:3812, in Index.get_loc(self, key)
   3811 try:
-> 3812     return self._engine.get_loc(casted_key)
   3813 except KeyError as err:

File pandas/_libs/index.pyx:167, in pandas._libs.index.IndexEngine.get_loc()

File pandas/_libs/index.pyx:196, in pandas._libs.index.IndexEngine.get_loc()

File pandas/_libs/hashtable_class_helper.pxi:7088, in pandas._libs.hashtable.PyObjectHashTable.get_item()

File pandas/_libs/hashtable_class_helper.pxi:7096, in pandas._libs.hashtable.PyObjectHashTable.get_item()

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/python3.13/site-packages/pandas/core/indexing.py:1191, in _LocationIndexer.__getitem__(self, key)
   1189 maybe_callable = com.apply_if_callable(key, self.obj)
   1190 maybe_callable = self._check_deprecated_callable_usage(key, maybe_callable)
-> 1191 return self._getitem_axis(maybe_callable, axis=axis)

File ~/My-Environments/jfasch-home/lib64/python3.13/site-packages/pandas/core/indexing.py:1431, in _LocIndexer._getitem_axis(self, key, axis)
   1429 # fall thru to straight lookup
   1430 self._validate_key(key, axis)
-> 1431 return self._get_label(key, axis=axis)

File ~/My-Environments/jfasch-home/lib64/python3.13/site-packages/pandas/core/indexing.py:1381, in _LocIndexer._get_label(self, label, axis)
   1379 def _get_label(self, label, axis: AxisInt):
   1380     # GH#5567 this will fail if the label is not present in the axis.
-> 1381     return self.obj.xs(label, axis=axis)

File ~/My-Environments/jfasch-home/lib64/python3.13/site-packages/pandas/core/generic.py:4320, in NDFrame.xs(self, key, axis, level, drop_level)
   4318             new_index = index[loc]
   4319 else:
-> 4320     loc = index.get_loc(key)
   4322     if isinstance(loc, np.ndarray):
   4323         if loc.dtype == np.bool_:

File ~/My-Environments/jfasch-home/lib64/python3.13/site-packages/pandas/core/indexes/base.py:3819, in Index.get_loc(self, key)
   3814     if isinstance(casted_key, slice) or (
   3815         isinstance(casted_key, abc.Iterable)
   3816         and any(isinstance(x, slice) for x in casted_key)
   3817     ):
   3818         raise InvalidIndexError(key)
-> 3819     raise KeyError(key) from err
   3820 except TypeError:
   3821     # If we have a listlike key, _check_indexing_error will raise
   3822     #  InvalidIndexError. Otherwise we fall through and re-raise
   3823     #  the TypeError.
   3824     self._check_indexing_error(key)

KeyError: 0

New row label: email

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

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

persons.loc[['jf@faschingbauer.co.at', 'johanna@email.com']]

	firstname	lastname	age
email
jf@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: jf@faschingbauer.co.at, dtype: object

persons.iloc[[0, 1]]

	firstname	lastname	age
email
jf@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
jf@faschingbauer.co.at	Joerg	Faschingbauer	56
johanna@email.com	Johanna	Faschingbauer	27
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
jf@faschingbauer.co.at	Joerg	Faschingbauer	56
johanna@email.com	Johanna	Faschingbauer	27
philipp@email.com	Philipp	Lichtenberger	37

Links #

Corey Schafer: Python Pandas Tutorial (Part 3): Indexes - How to Set, Reset, and Use Indexes
Data School: How do I use the MultiIndex in pandas?

Pandas: Indexes#

Default Index: Row Number#

Setting Custom Index#

Setting Custom Index, inplace=True#

Custom Index, And loc[]#

Custom Index, And iloc[]#

Sorting DataFrame Object By Index Column#

Links#