Pandas provides data structures for in-memory analytics, which makes using pandas to analyze datasets that are larger than memory datasets somewhat tricky. Even datasets that are a sizable fraction of memory become unwieldy, as some pandas operations need to make intermediate copies.
This document provides a few recommendations for scaling your analysis to larger datasets. It’s a complement to Enhancing performance, which focuses on speeding up analysis for datasets that fit in memory.
But first, it’s worth considering not using pandas. Pandas isn’t the right tool for all situations. If you’re working with very large datasets and a tool like PostgreSQL fits your needs, then you should probably be using that. Assuming you want or need the expressiveness and power of pandas, let’s carry on.
In [1]: import pandas as pd In [2]: import numpy as np
Suppose our raw dataset on disk has many columns:
id_0 name_0 x_0 y_0 id_1 name_1 x_1 ... name_8 x_8 y_8 id_9 name_9 x_9 y_9 timestamp ... 2000-01-01 00:00:00 1015 Michael -0.399453 0.095427 994 Frank -0.176842 ... Dan -0.315310 0.713892 1025 Victor -0.135779 0.346801 2000-01-01 00:01:00 969 Patricia 0.650773 -0.874275 1003 Laura 0.459153 ... Ursula 0.913244 -0.630308 1047 Wendy -0.886285 0.035852 2000-01-01 00:02:00 1016 Victor -0.721465 -0.584710 1046 Michael 0.524994 ... Ray -0.656593 0.692568 1064 Yvonne 0.070426 0.432047 2000-01-01 00:03:00 939 Alice -0.746004 -0.908008 996 Ingrid -0.414523 ... Jerry -0.958994 0.608210 978 Wendy 0.855949 -0.648988 2000-01-01 00:04:00 1017 Dan 0.919451 -0.803504 1048 Jerry -0.569235 ... Frank -0.577022 -0.409088 994 Bob -0.270132 0.335176 ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... 2000-12-30 23:56:00 999 Tim 0.162578 0.512817 973 Kevin -0.403352 ... Tim -0.380415 0.008097 1041 Charlie 0.191477 -0.599519 2000-12-30 23:57:00 970 Laura -0.433586 -0.600289 958 Oliver -0.966577 ... Zelda 0.971274 0.402032 1038 Ursula 0.574016 -0.930992 2000-12-30 23:58:00 1065 Edith 0.232211 -0.454540 971 Tim 0.158484 ... Alice -0.222079 -0.919274 1022 Dan 0.031345 -0.657755 2000-12-30 23:59:00 1019 Ingrid 0.322208 -0.615974 981 Hannah 0.607517 ... Sarah -0.424440 -0.117274 990 George -0.375530 0.563312 2000-12-31 00:00:00 937 Ursula -0.906523 0.943178 1018 Alice -0.564513 ... Jerry 0.236837 0.807650 985 Oliver 0.777642 0.783392 [525601 rows x 40 columns]
To load the columns we want, we have two options. Option 1 loads in all the data and then filters to what we need.
In [3]: columns = ['id_0', 'name_0', 'x_0', 'y_0'] In [4]: pd.read_parquet("timeseries_wide.parquet")[columns] Out[4]: id_0 name_0 x_0 y_0 timestamp 2000-01-01 00:00:00 1015 Michael -0.399453 0.095427 2000-01-01 00:01:00 969 Patricia 0.650773 -0.874275 2000-01-01 00:02:00 1016 Victor -0.721465 -0.584710 2000-01-01 00:03:00 939 Alice -0.746004 -0.908008 2000-01-01 00:04:00 1017 Dan 0.919451 -0.803504 ... ... ... ... ... 2000-12-30 23:56:00 999 Tim 0.162578 0.512817 2000-12-30 23:57:00 970 Laura -0.433586 -0.600289 2000-12-30 23:58:00 1065 Edith 0.232211 -0.454540 2000-12-30 23:59:00 1019 Ingrid 0.322208 -0.615974 2000-12-31 00:00:00 937 Ursula -0.906523 0.943178 [525601 rows x 4 columns]
Option 2 only loads the columns we request.
In [5]: pd.read_parquet("timeseries_wide.parquet", columns=columns) Out[5]: id_0 name_0 x_0 y_0 timestamp 2000-01-01 00:00:00 1015 Michael -0.399453 0.095427 2000-01-01 00:01:00 969 Patricia 0.650773 -0.874275 2000-01-01 00:02:00 1016 Victor -0.721465 -0.584710 2000-01-01 00:03:00 939 Alice -0.746004 -0.908008 2000-01-01 00:04:00 1017 Dan 0.919451 -0.803504 ... ... ... ... ... 2000-12-30 23:56:00 999 Tim 0.162578 0.512817 2000-12-30 23:57:00 970 Laura -0.433586 -0.600289 2000-12-30 23:58:00 1065 Edith 0.232211 -0.454540 2000-12-30 23:59:00 1019 Ingrid 0.322208 -0.615974 2000-12-31 00:00:00 937 Ursula -0.906523 0.943178 [525601 rows x 4 columns]
If we were to measure the memory usage of the two calls, we’d see that specifying columns uses about 1/10th the memory in this case.
columns
With pandas.read_csv(), you can specify usecols to limit the columns read into memory. Not all file formats that can be read by pandas provide an option to read a subset of columns.
pandas.read_csv()
usecols
The default pandas data types are not the most memory efficient. This is especially true for text data columns with relatively few unique values (commonly referred to as “low-cardinality” data). By using more efficient data types, you can store larger datasets in memory.
In [6]: ts = pd.read_parquet("timeseries.parquet") In [7]: ts Out[7]: id name x y timestamp 2000-01-01 00:00:00 1029 Michael 0.278837 0.247932 2000-01-01 00:00:30 1010 Patricia 0.077144 0.490260 2000-01-01 00:01:00 1001 Victor 0.214525 0.258635 2000-01-01 00:01:30 1018 Alice -0.646866 0.822104 2000-01-01 00:02:00 991 Dan 0.902389 0.466665 ... ... ... ... ... 2000-12-30 23:58:00 992 Sarah 0.721155 0.944118 2000-12-30 23:58:30 1007 Ursula 0.409277 0.133227 2000-12-30 23:59:00 1009 Hannah -0.452802 0.184318 2000-12-30 23:59:30 978 Kevin -0.904728 -0.179146 2000-12-31 00:00:00 973 Ingrid -0.370763 -0.794667 [1051201 rows x 4 columns]
Now, let’s inspect the data types and memory usage to see where we should focus our attention.
In [8]: ts.dtypes Out[8]: id int64 name object x float64 y float64 dtype: object
In [9]: ts.memory_usage(deep=True) # memory usage in bytes Out[9]: Index 8409608 id 8409608 name 65537768 x 8409608 y 8409608 dtype: int64
The name column is taking up much more memory than any other. It has just a few unique values, so it’s a good candidate for converting to a Categorical. With a Categorical, we store each unique name once and use space-efficient integers to know which specific name is used in each row.
name
Categorical
In [10]: ts2 = ts.copy() In [11]: ts2['name'] = ts2['name'].astype('category') In [12]: ts2.memory_usage(deep=True) Out[12]: Index 8409608 id 8409608 name 1054102 x 8409608 y 8409608 dtype: int64
We can go a bit further and downcast the numeric columns to their smallest types using pandas.to_numeric().
pandas.to_numeric()
In [13]: ts2['id'] = pd.to_numeric(ts2['id'], downcast='unsigned') In [14]: ts2[['x', 'y']] = ts2[['x', 'y']].apply(pd.to_numeric, downcast='float') In [15]: ts2.dtypes Out[15]: id uint16 name category x float32 y float32 dtype: object
In [16]: ts2.memory_usage(deep=True) Out[16]: Index 8409608 id 2102402 name 1054102 x 4204804 y 4204804 dtype: int64
In [17]: reduction = (ts2.memory_usage(deep=True).sum() ....: / ts.memory_usage(deep=True).sum()) ....: In [18]: print(f"{reduction:0.2f}") 0.20
In all, we’ve reduced the in-memory footprint of this dataset to 1/5 of its original size.
See Categorical data for more on Categorical and dtypes for an overview of all of pandas’ dtypes.
Some workloads can be achieved with chunking: splitting a large problem like “convert this directory of CSVs to parquet” into a bunch of small problems (“convert this individual CSV file into a Parquet file. Now repeat that for each file in this directory.”). As long as each chunk fits in memory, you can work with datasets that are much larger than memory.
Note
Chunking works well when the operation you’re performing requires zero or minimal coordination between chunks. For more complicated workflows, you’re better off using another library.
Suppose we have an even larger “logical dataset” on disk that’s a directory of parquet files. Each file in the directory represents a different year of the entire dataset.
data └── timeseries ├── ts-00.parquet ├── ts-01.parquet ├── ts-02.parquet ├── ts-03.parquet ├── ts-04.parquet ├── ts-05.parquet ├── ts-06.parquet ├── ts-07.parquet ├── ts-08.parquet ├── ts-09.parquet ├── ts-10.parquet └── ts-11.parquet
Now we’ll implement an out-of-core value_counts. The peak memory usage of this workflow is the single largest chunk, plus a small series storing the unique value counts up to this point. As long as each individual file fits in memory, this will work for arbitrary-sized datasets.
value_counts
In [19]: %%time ....: files = pathlib.Path("data/timeseries/").glob("ts*.parquet") ....: counts = pd.Series(dtype=int) ....: for path in files: ....: df = pd.read_parquet(path) ....: counts = counts.add(df['name'].value_counts(), fill_value=0) ....: counts.astype(int) ....: CPU times: user 1.98 s, sys: 309 ms, total: 2.29 s Wall time: 2.23 s Out[19]: Alice 229802 Bob 229211 Charlie 229303 Dan 230621 Edith 230349 ... Victor 230502 Wendy 230038 Xavier 229553 Yvonne 228766 Zelda 229909 Length: 26, dtype: int64
Some readers, like pandas.read_csv(), offer parameters to control the chunksize when reading a single file.
chunksize
Manually chunking is an OK option for workflows that don’t require too sophisticated of operations. Some operations, like groupby, are much harder to do chunkwise. In these cases, you may be better switching to a different library that implements these out-of-core algorithms for you.
groupby
Pandas is just one library offering a DataFrame API. Because of its popularity, pandas’ API has become something of a standard that other libraries implement. The pandas documentation maintains a list of libraries implementing a DataFrame API in our ecosystem page.
For example, Dask, a parallel computing library, has dask.dataframe, a pandas-like API for working with larger than memory datasets in parallel. Dask can use multiple threads or processes on a single machine, or a cluster of machines to process data in parallel.
We’ll import dask.dataframe and notice that the API feels similar to pandas. We can use Dask’s read_parquet function, but provide a globstring of files to read in.
dask.dataframe
read_parquet
In [20]: import dask.dataframe as dd In [21]: ddf = dd.read_parquet("data/timeseries/ts*.parquet", engine="pyarrow") In [22]: ddf Out[22]: Dask DataFrame Structure: id name x y npartitions=12 int64 object float64 float64 ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... Dask Name: read-parquet, 12 tasks
Inspecting the ddf object, we see a few things
ddf
There are familiar attributes like .columns and .dtypes
.columns
.dtypes
There are familiar methods like .groupby, .sum, etc.
.groupby
.sum
There are new attributes like .npartitions and .divisions
.npartitions
.divisions
The partitions and divisions are how Dask parallizes computation. A Dask DataFrame is made up of many Pandas DataFrames. A single method call on a Dask DataFrame ends up making many pandas method calls, and Dask knows how to coordinate everything to get the result.
In [23]: ddf.columns Out[23]: Index(['id', 'name', 'x', 'y'], dtype='object') In [24]: ddf.dtypes Out[24]: id int64 name object x float64 y float64 dtype: object In [25]: ddf.npartitions Out[25]: 12
One major difference: the dask.dataframe API is lazy. If you look at the repr above, you’ll notice that the values aren’t actually printed out; just the column names and dtypes. That’s because Dask hasn’t actually read the data yet. Rather than executing immediately, doing operations build up a task graph.
In [26]: ddf Out[26]: Dask DataFrame Structure: id name x y npartitions=12 int64 object float64 float64 ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... Dask Name: read-parquet, 12 tasks In [27]: ddf['name'] Out[27]: Dask Series Structure: npartitions=12 object ... ... ... ... Name: name, dtype: object Dask Name: getitem, 24 tasks In [28]: ddf['name'].value_counts() Out[28]: Dask Series Structure: npartitions=1 int64 ... Name: name, dtype: int64 Dask Name: value-counts-agg, 39 tasks
Each of these calls is instant because the result isn’t being computed yet. We’re just building up a list of computation to do when someone needs the result. Dask knows that the return type of a pandas.Series.value_counts is a pandas Series with a certain dtype and a certain name. So the Dask version returns a Dask Series with the same dtype and the same name.
pandas.Series.value_counts
To get the actual result you can call .compute().
.compute()
In [29]: %time ddf['name'].value_counts().compute() CPU times: user 1.52 s, sys: 191 ms, total: 1.71 s Wall time: 1.49 s Out[29]: Laura 230906 Ingrid 230838 Kevin 230698 Dan 230621 Frank 230595 ... Ray 229603 Xavier 229553 Charlie 229303 Bob 229211 Yvonne 228766 Name: name, Length: 26, dtype: int64
At that point, you get back the same thing you’d get with pandas, in this case a concrete pandas Series with the count of each name.
Calling .compute causes the full task graph to be executed. This includes reading the data, selecting the columns, and doing the value_counts. The execution is done in parallel where possible, and Dask tries to keep the overall memory footprint small. You can work with datasets that are much larger than memory, as long as each partition (a regular pandas DataFrame) fits in memory.
.compute
By default, dask.dataframe operations use a threadpool to do operations in parallel. We can also connect to a cluster to distribute the work on many machines. In this case we’ll connect to a local “cluster” made up of several processes on this single machine.
>>> from dask.distributed import Client, LocalCluster >>> cluster = LocalCluster() >>> client = Client(cluster) >>> client <Client: 'tcp://127.0.0.1:53349' processes=4 threads=8, memory=17.18 GB>
Once this client is created, all of Dask’s computation will take place on the cluster (which is just processes in this case).
client
Dask implements the most used parts of the pandas API. For example, we can do a familiar groupby aggregation.
In [30]: %time ddf.groupby('name')[['x', 'y']].mean().compute().head() CPU times: user 2.5 s, sys: 1.68 s, total: 4.18 s Wall time: 2.72 s Out[30]: x y name Alice 0.000086 -0.001170 Bob -0.000843 -0.000799 Charlie 0.000564 -0.000038 Dan 0.000584 0.000818 Edith -0.000116 -0.000044
The grouping and aggregation is done out-of-core and in parallel.
When Dask knows the divisions of a dataset, certain optimizations are possible. When reading parquet datasets written by dask, the divisions will be known automatically. In this case, since we created the parquet files manually, we need to supply the divisions manually.
divisions
In [31]: N = 12 In [32]: starts = [f'20{i:>02d}-01-01' for i in range(N)] In [33]: ends = [f'20{i:>02d}-12-13' for i in range(N)] In [34]: divisions = tuple(pd.to_datetime(starts)) + (pd.Timestamp(ends[-1]),) In [35]: ddf.divisions = divisions In [36]: ddf Out[36]: Dask DataFrame Structure: id name x y npartitions=12 2000-01-01 int64 object float64 float64 2001-01-01 ... ... ... ... ... ... ... ... ... 2011-01-01 ... ... ... ... 2011-12-13 ... ... ... ... Dask Name: read-parquet, 12 tasks
Now we can do things like fast random access with .loc.
.loc
In [37]: ddf.loc['2002-01-01 12:01':'2002-01-01 12:05'].compute() Out[37]: id name x y timestamp 2002-01-01 12:01:00 983 Laura 0.243985 -0.079392 2002-01-01 12:02:00 1001 Laura -0.523119 -0.226026 2002-01-01 12:03:00 1059 Oliver 0.612886 0.405680 2002-01-01 12:04:00 993 Kevin 0.451977 0.332947 2002-01-01 12:05:00 1014 Yvonne -0.948681 0.361748
Dask knows to just look in the 3rd partition for selecting values in 2002. It doesn’t need to look at any other data.
Many workflows involve a large amount of data and processing it in a way that reduces the size to something that fits in memory. In this case, we’ll resample to daily frequency and take the mean. Once we’ve taken the mean, we know the results will fit in memory, so we can safely call compute without running out of memory. At that point it’s just a regular pandas object.
compute
In [38]: ddf[['x', 'y']].resample("1D").mean().cumsum().compute().plot() Out[38]: <matplotlib.axes._subplots.AxesSubplot at 0x7f916881f910>
These Dask examples have all be done using multiple processes on a single machine. Dask can be deployed on a cluster to scale up to even larger datasets.
You see more dask examples at https://examples.dask.org.