python – pandas .at versus .loc-ThrowExceptions

Exception or error:

I’ve been exploring how to optimize my code and ran across pandas .at method. Per the documentation

Fast label-based scalar accessor

Similarly to loc, at provides label based scalar lookups. You can also set using these indexers.

So I ran some samples:

Setup

import pandas as pd
import numpy as np
from string import letters, lowercase, uppercase

lt = list(letters)
lc = list(lowercase)
uc = list(uppercase)

def gdf(rows, cols, seed=None):
    """rows and cols are what you'd pass
    to pd.MultiIndex.from_product()"""
    gmi = pd.MultiIndex.from_product
    df = pd.DataFrame(index=gmi(rows), columns=gmi(cols))
    np.random.seed(seed)
    df.iloc[:, :] = np.random.rand(*df.shape)
    return df

seed = [3, 1415]
df = gdf([lc, uc], [lc, uc], seed)

print df.head().T.head().T

df looks like:

            a                                        
            A         B         C         D         E
a A  0.444939  0.407554  0.460148  0.465239  0.462691
  B  0.032746  0.485650  0.503892  0.351520  0.061569
  C  0.777350  0.047677  0.250667  0.602878  0.570528
  D  0.927783  0.653868  0.381103  0.959544  0.033253
  E  0.191985  0.304597  0.195106  0.370921  0.631576

Lets use .at and .loc and ensure I get the same thing

print "using .loc", df.loc[('a', 'A'), ('c', 'C')]
print "using .at ", df.at[('a', 'A'), ('c', 'C')]

using .loc 0.37374090276
using .at  0.37374090276

Test speed using .loc

%%timeit
df.loc[('a', 'A'), ('c', 'C')]

10000 loops, best of 3: 180 µs per loop

Test speed using .at

%%timeit
df.at[('a', 'A'), ('c', 'C')]

The slowest run took 6.11 times longer than the fastest. This could mean that an intermediate result is being cached.
100000 loops, best of 3: 8 µs per loop

This looks to be a huge speed increase. Even at the caching stage 6.11 * 8 is a lot faster than 180

Question

What are the limitations of .at? I’m motivated to use it. The documentation says it’s similar to .loc but it doesn’t behave similarly. Example:

# small df
sdf = gdf([lc[:2]], [uc[:2]], seed)

print sdf.loc[:, :]

          A         B
a  0.444939  0.407554
b  0.460148  0.465239

where as print sdf.at[:, :] results in TypeError: unhashable type

So obviously not the same even if the intent is to be similar.

That said, who can provide guidance on what can and cannot be done with the .at method?

How to solve:

Update: df.get_value is deprecated as of version 0.21.0. Using df.at or df.iat is the recommended method going forward.


df.at can only access a single value at a time.

df.loc can select multiple rows and/or columns.

Note that there is also df.get_value, which may be even quicker at accessing single values:

In [25]: %timeit df.loc[('a', 'A'), ('c', 'C')]
10000 loops, best of 3: 187 µs per loop

In [26]: %timeit df.at[('a', 'A'), ('c', 'C')]
100000 loops, best of 3: 8.33 µs per loop

In [35]: %timeit df.get_value(('a', 'A'), ('c', 'C'))
100000 loops, best of 3: 3.62 µs per loop

Under the hood, df.at[...] calls df.get_value, but it also does some type checking on the keys.

Answer:

As you asked about the limitations of .at, here is one thing I recently ran into (using pandas 0.22). Let’s use the example from the documentation:

df = pd.DataFrame([[0, 2, 3], [0, 4, 1], [10, 20, 30]], index=[4, 5, 6], columns=['A', 'B', 'C'])
df2 = df.copy()

    A   B   C
4   0   2   3
5   0   4   1
6  10  20  30

If I now do

df.at[4, 'B'] = 100

the result looks as expected

    A    B   C
4   0  100   3
5   0    4   1
6  10   20  30

However, when I try to do

 df.at[4, 'C'] = 10.05

it seems that .at tries to conserve the datatype (here: int):

    A    B   C
4   0  100  10
5   0    4   1
6  10   20  30

That seems to be a difference to .loc:

df2.loc[4, 'C'] = 10.05

yields the desired

    A   B      C
4   0   2  10.05
5   0   4   1.00
6  10  20  30.00

The risky thing in the example above is that it happens silently (the conversion from float to int). When one tries the same with strings it will throw an error:

df.at[5, 'A'] = 'a_string'

ValueError: invalid literal for int() with base 10: ‘a_string’

It will work, however, if one uses a string on which int() actually works as noted by @n1k31t4 in the comments, e.g.

df.at[5, 'A'] = '123'

     A   B   C
4    0   2   3
5  123   4   1
6   10  20  30

Answer:

.at is an optimized data access method compared to .loc .

.loc of a data frame selects all the elements located by indexed_rows and labeled_columns as given in its argument. Insetad, .at selects particular elemnt of a data frame positioned at the given indexed_row and labeled_column.

Also, .at takes one row and one column as input argument, whereas .loc may take multiple rows and columns. Oputput using .at is a single element and using .loc maybe a Series or a DataFrame.

Leave a Reply

Your email address will not be published. Required fields are marked *