 1. Introduction
 2. Setup
 3. Aggregate Heaven
 4. Side Note: Data Types
 5. More About Aggregates
 6. Aggregate performance
 7. Conclusion
1. Introduction
The other day I caught myself writing one too many df.groupby()
in Pandas, and thought to myself: Isn’t there a way I can simplify and reduce the amount of df.groupby()
calls? What I group by hardly changes and only the operation that follows a group by varies from IPython cell to IPython cell. Aggregates shall come to the rescue.
Pandas DataFrames allow many useful calculations to be performed on them. One of them is the so called .aggregate()
method. Using .aggregate()
, a user can perform many calculations on a group by object or resampler at once. This can be quite handy in many situations and performs much faster than calculating all required aggregate values in separate steps. As a matter of fact, I was quite surprised while writing this article when I found out how much flexibility exactly a user can get while using aggregate()
.
2. Setup
First things first and we do business as usual. We import pandas
in order to create the DataFrames we use in our examples.
import pandas as pd
We want to define a DataFrame with a variety of illustrative data types.
The example that we come up with here is a list of fruits that our fictitious friends Franz, Hans and Gerhard have eaten in the last week. Furthermore, we note down whether our friends have actually liked the fruit or not.
So the columns are:

index
: Name of person 
Fruit
: Fruit consumed 
Satisfaction
: Satisfaction with consumed fruit 
Weight
: Weight of the consumed fruit in grams
df = pd.DataFrame(
[
['Apple', 'full', 100],
['Orange', 'none', 200],
['Pear', 'full', 300],
['Pear', 'partial', 100],
['Banana', 'full', 400],
['Banana', 'full', 300],
],
columns=[
'Fruit',
'Satisfaction',
'Weight'
],
index=[
'Franz',
'Gerhard',
'Gerhard',
'Hans',
'Hans',
'Hans',
],
)
df.Satisfaction = df.Satisfaction.astype('category')
df
Output:
Fruit  Satisfaction  Weight  

Franz  Apple  full  100 
Gerhard  Orange  none  200 
Gerhard  Pear  full  300 
Hans  Pear  partial  100 
Hans  Banana  full  400 
Hans  Banana  full  300 
Are you as excited as I am to learn the first few magic incantations of .aggregate()
? Let’s move on and start working on the DataFrame.
3. Aggregate Heaven
count
3.1. Using the count
aggregation, we can retrieve the amount of rows that are counted in a group by expression. This is not particularly exciting, but will make the following steps a little bit clearer.
First we turn towards our Pandas DataFrame and try to count the number of fruits that each person has consumed.
For this, we have to group the DataFrame by its index as the index contains the person names. In order to group by the DataFrame’s index, we can simply group by using .groupby(level=0)
. We use level=0
to indicate that we want to group by the DataFrames index. We could use .groupby(df.index)
instead, but this way we can leave it implicit and save us a bit of typing. See the docs for some more information on how groupby()
can be invoked.
df.groupby(level=0)
Output:
<pandas.core.groupby.DataFrameGroupBy object at 0x10b88ada0>
Calling .groupby()
by itself does not do much. We need to perform an aggregation on it in order to get a meaningful result. Let’s do the actual calculation now and see what we get. We call .aggregate('count')
on the DataFrameGroupBy
object.
df.groupby(level=0).aggregate('count')
Output:
Fruit  Satisfaction  Weight  

Franz  1  1  1 
Gerhard  2  2  2 
Hans  3  3  3 
Our informed reader will surely let us know, that count()
can also be invoked more directly and will give us the same result:
df.groupby(level=0).count()
Output:
Fruit  Satisfaction  Weight  

Franz  1  1  1 
Gerhard  2  2  2 
Hans  3  3  3 
That is absolutely true, and in simple situations just using .count()
on a group by object will be much easier to understand. On the other hand, aggregate()
allows us to do one nifty thing that wouldn’t be possible easily any other way. It lets us perform several calculations at once and neatly formats the results using nested columns. As a matter of fact, Pandas gives us a lot of freedom in how exactly we want the aggregates to look like. Let’s get to a slightly more complicated example to illustrate the true flexibility right away.
3.2. Aggregator Functions
If we would like to find out what the most frequent value in a column is, we need to use a custom aggregator. Pandas does not include a method for this out of the box, so we can either define a function or a lambda to give us the desired result.
In our case we would like to simply define a lambda to facilitate this calculation:
most_frequent = lambda s: s.value_counts().idxmax()
We can try calling most_frequent
on the whole DataFrame and check the result. We use .apply()
in this case to apply a function to every column in a DataFrame.
df.apply(most_frequent).to_frame()
Output:
0  

Fruit  Banana 
Satisfaction  full 
Weight  100 
Let’s put everything together and calculate the aggregate.
df.groupby(level=0).aggregate(lambda s: s.value_counts().idxmax())
Output:
Fruit  Satisfaction  Weight  

Franz  Apple  full  100 
Gerhard  Pear  none  300 
Hans  Banana  full  400 
3.3. Combining Aggregates
Let us find out, what
 the first and last fruit that each person has eaten is,
 while also counting the total amount of fruits consumed, and
 what the most frequently given satisfaction rating for each person is.
In order to do this, we first have to define an aggregate dictionary. It contains instructions on which calculations to perform on which column. It even allows defining custom aggregators using Python functions or lambdas.
The aggregate dictionary contains two entries corresponding to the two columns in the DataFrame. For the fruit column, we add 3 desired aggregates in the form of a list:
['first', 'last', 'count']
And for the satisfaction column, we add a named aggregator function by specifying a list containing one tuple with the most_frequent
lambda that we have defined before:
[('most_frequent', most_frequent)]
Attaching a name to the aggregator is useful in our case, since it will tell Pandas what to name the result column after calculating the aggregates. Otherwise, using a lambda will lead to the resulting column to also be called lambda
.
The full dictionary is defined as follows:
aggregate = {
'Fruit': [
'first',
'last',
'count',
],
'Satisfaction': [
('most_frequent', most_frequent),
]
}
Let’s run the actual calculation then and see what the result looks like.
df.groupby(level=0).aggregate(aggregate)
Output:
Fruit  Satisfaction  

first  last  count  most_frequent  
Franz  Apple  Apple  1  full 
Gerhard  Orange  Pear  2  none 
Hans  Pear  Banana  3  full 
We can see that the DataFrame’s columns are formatted and calculated exactly as we have defined in the dictionary used for our .aggregate()
call.
4. Side Note: Data Types
4.1. Pandas dtypes
Note that when running aggregates, the result type of an aggregated column can be different from the source column. To get back to our count()
example, observe the following data types for the source DataFrame:
df.dtypes.to_frame()
Output:
0  

Fruit  object 
Satisfaction  category 
Weight  int64 
 The
Fruit
column contains data of the typeobject
, which is the way Pandas stores Pythonstr
(string) data in columns, 
Satisfaction
contains category data, as indicated previously, and 
Weight
containsint64
data.
Now, observe one more time what happens when the count aggregate is retrieved on the same DataFrame.
df.groupby(level=0).aggregate('count')
Output:
Fruit  Satisfaction  Weight  

Franz  1  1  1 
Gerhard  2  2  2 
Hans  3  3  3 
We look at the .dtypes
attribute of our aggregate.
df.groupby(level=0).aggregate('count').dtypes.to_frame()
Output:
0  

Fruit  int64 
Satisfaction  int64 
Weight  int64 
This reveals that count
will be stored as int64
unlike the original columns, which had the data types object
and category
. Pandas returns a different data type after performing aggregates, depending on what the result of a calculation is.
In the case of count
data, integers such as int64
(a 64 bit signed integer) are the sensible choice for storing them. NumPy ndarray
is the internal storage format used for almost all data in Pandas, except for indices. More info on ndarray
can be found here. The reason why Pandas uses ndarray
objects is that it is a space and time efficient way of storing fixed length lists of numbers.
We can easily take a look at the underlying data type of a Pandas column by accessing the .values
attribute.
type(df.Fruit.values)
Output:
numpy.ndarray
And simply evaluating df.Fruit.values
reveals that it has the data type object
, as is indicated by the dtype
value.
df.Fruit.values
Output:
array(['Apple', 'Orange', 'Pear', 'Pear', 'Banana', 'Banana'], dtype=object)
object
is used to store str
(string) data in NumPy and Pandas. Unlike fixedwidth integers, such as int64
, string data cannot be stored efficiently inside a continuous NumPy ndarray
. The ndarray
instance in this case only contains a collection of pointers to objects, quite similarly to how a regular Python list
works. The space efficiency characteristics of a ndarray
instance with data type set to object
are slightly better than just using list
. We will prove this in the next subsection.
4.2. Memory Profiling
Our claim that NumPy ndarrays store strings more efficiently than Python lists can be verified using a memory profiler. Luckily, a PyPi package called pympler
allows us to quickly measured memory usage by Python objects. We furthermore import NumPy to directly create NumPy arrays without requiring Pandas.
from pympler import asizeof
import numpy as np
We decide to create an ndarray
containing 10**6
strings “hello”.
numpy_hello = np.zeros(10 ** 6, dtype=object)
numpy_hello.fill('hello')
numpy_hello
Output:
array(['hello', 'hello', 'hello', ..., 'hello', 'hello', 'hello'], dtype=object)
We create an equivalent Python list
that contains “hello” 10**6
times.
python_hello = ["hello" for _ in range(10 ** 6)]
# Print the first 3 items
python_hello[:3]
Output:
['hello', 'hello', 'hello']
Now for the evaluation: We print the size of the NumPy ndarray
in bytes using Pympler’s asizeof
method.
asizeof.asizeof(numpy_hello)
Output:
8000096
To compare, we print the size of the Python list
.
asizeof.asizeof(python_hello)
Output:
8697520
print("NumPy array size in relation to Python list size: {:2.2%}".format(
asizeof.asizeof(numpy_hello) /
asizeof.asizeof(python_hello)
))
Output:
NumPy array size in relation to Python list size: 91.98%
The NumPy array only takes 91.98% of the space required by the Python list, even though the data that they store is the same.
About Pympler
Note that Pympler prints accurate sizes by traversing the object and summing up attribute sizes for all descendant attributes. This is unlike sys.getsizeof
, which does not perform a deep traversal of an object and its attributes, especially for user defined classes.
We can easily see the difference:
from sys import getsizeof
getsizeof(python_hello)
Output:
8697464
Now here the difference is minimal, but as soon as we nest objects even further, the difference of sys.getsizeof
and asizeof
becomes quite obvious:
getsizeof([[[]]])
Output:
72
asizeof.asizeof([[[]]])
Output:
208
As a bonus, we compare the previous two objects numpy_hello
and python_hello
to using a tuple()
instead.
tuple_hello = tuple("hello" for _ in range(10 ** 6))
# Print the first 3 items
tuple_hello[:3]
Output:
('hello', 'hello', 'hello')
tuple_hello
is smaller than the list object python_hello
, but still bigger than numpy_hello
:
asizeof.asizeof(tuple_hello)
Output:
8000104
From this we can safely conclude that Numpy ndarray
is the most efficient way of storing fixedsize array data.
This concludes our short excursion on Python and NumPy memory usage.
5. More About Aggregates
nunique
5.1. In Pandas, nunique
counts the number of unique values in a column. We can apply this to the whole DataFrame and get a count of the unique fruit and satisfaction values:
df.nunique().to_frame()
Output:
0  

Fruit  4 
Satisfaction  3 
Weight  4 
Furthermore, the method can also be applied on a group by object to retrieve the unique number of values per group. We see below the number of unique fruits and satisfactions that have been assigned to each person.
df.groupby(level=0).nunique()
Output:
Fruit  Satisfaction  Weight  

Franz  1  1  1 
Gerhard  2  2  2 
Hans  2  2  3 
Now, nunique
is also available in aggregates. The reason why we would use nunique
in aggregates is if we want to retrieve multiple results for one group by expression at the same time. This can not only save us some typing, but can potentially also save us some computational time, as a group only needs to be created once and each operation can then be applied to it one after another.
df.groupby(level=0).agg('nunique')
Output:
Fruit  Satisfaction  Weight  

Franz  1  1  1 
Gerhard  2  2  2 
Hans  2  2  3 
5.2. Using Aggregate Results
Having run this, we now know that Franz has only consumed one type of fruit. Hans is the champion of trying out many types of fruits. We can then use this value to compare it to the total count of fruits consumed. This allows us to calculate a variety score for each person. We define a variety of 100 % as a fruit consumption pattern in which a new fruit is tried every time.
fruit_counts = df.groupby(level=0).Fruit.aggregate(['nunique', 'count'])
fruit_counts
Output:
nunique  count  

Franz  1  1 
Gerhard  2  2 
Hans  2  3 
We decide to neatly display the variety counts with a quick .apply
call in which we format the resulting floats using a Python format string.
(
fruit_counts['nunique'] / fruit_counts['count']
).apply(
lambda v: "{:.2%}".format(v)
).to_frame('Variety')
Output:
Variety  

Franz  100.00% 
Gerhard  100.00% 
Hans  66.67% 
5.3. All BuiltIn Aggregates
What follows now is a list of all the builtin aggregates that I could find in Pandas.
These are the aggregates that work with all data types. To save some space we limit ourselves to the Fruit column.
df.groupby(level=0).Fruit.aggregate([
'count',
'min',
'max',
'first',
'last',
'nunique',
]).applymap(
lambda v: v if isinstance(v, str) else "{:d}".format(v)
)
Output:
count  min  max  first  last  nunique  

Franz  1  Apple  Apple  Apple  Apple  1 
Gerhard  2  Orange  Pear  Orange  Pear  2 
Hans  3  Banana  Pear  Pear  Banana  2 
Here are the data types that work with numerical data types.
df.groupby(level=0).aggregate([
'mean',
'std',
'var',
'median',
'prod',
'sum',
'mad',
'sem',
'skew',
'quantile', # 50 % quantile
]).applymap(
lambda v: v if isinstance(v, str) else "{:.2f}".format(v)
)
Output:
Weight  

mean  std  var  median  prod  sum  mad  sem  skew  quantile  
Franz  100.00  nan  nan  100.00  100.00  100.00  0.00  nan  nan  100.00 
Gerhard  250.00  70.71  5000.00  250.00  60000.00  500.00  50.00  50.00  nan  250.00 
Hans  266.67  152.75  23333.33  300.00  12000000.00  800.00  111.11  88.19  0.94  300.00 
6. Aggregate performance
Let’s put the claim to the test that .aggregate()
calls are faster than aggregating directly on a group by, and find out in which circumstances this statement holds. We would first like to find out which is faster: Multiple aggregates in one .aggregate()
call, or separate aggregates applied to a new group by object each time? We choose to aggregate min
and max
on our DataFrame.
%%timeit
df.groupby(df.index).min()
df.groupby(df.index).max()
Output:
26.1 ms ± 819 µs per loop (mean ± std. dev. of 7 runs, 10 loops each)
%%timeit
df.groupby(df.index).aggregate(['min', 'max'])
Output:
9.02 ms ± 75.6 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)
We can clearly see that calling .aggregate()
performs much faster when multiple aggregate values are needed compared to aggregating twice on a .groupby()
. Furthermore, we can observe in the next two cells that there is hardly any difference when only one aggregate value is required. In this case, the shorter amount of code should win the contest, since it simply requires less typing.
%%timeit
df.groupby(df.index).min()
Output:
13.6 ms ± 945 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)
%%timeit
df.groupby(df.index).aggregate('min')
Output:
16.5 ms ± 4.14 ms per loop (mean ± std. dev. of 7 runs, 100 loops each)
7. Conclusion
I hope I was able to demonstrate a few use cases for Pandas aggregates. It is certainly nice to be able to save some typing and have better performance when dealing with multiple aggregate calculations, especially in a group by setting.
I certainly could not do without .aggregate()
, as it saves me a lot of time when typing out IPython notebooks.