6.12. Better Pandas#

This section cover tools to make your experience with Pandas a litte bit better.

6.12.1. tqdm: Add Progress Bar to Your Pandas Apply#

Hide code cell content
!pip install tqdm 

If you want to keep informed about the progress of a pandas apply operation, use tqdm.

import pandas as pd 
from tqdm import tqdm 
import time 

df = pd.DataFrame({'a': [1, 2, 3, 4, 5], 'b': [2, 3, 4, 5, 6]})

tqdm.pandas()
def func(row):
    time.sleep(1)
    return row + 1

df['a'].progress_apply(func)
100%|β–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆβ–ˆ| 5/5 [00:05<00:00,  1.00s/it]
0    2
1    3
2    4
3    5
4    6
Name: a, dtype: int64

Link to tqdm.

6.12.2. pandarallel: A Simple Tool to Parallelize Pandas Operations#

Hide code cell content
!pip install pandarallel

If you want to parallelize your Pandas operations on all available CPUs by adding only one line of code, try pandarallel.

from pandarallel import pandarallel
import pandas as pd
from numpy.random import randint

df = pd.DataFrame(
    {
        "a": randint(0, 100, size=10000),
        "b": randint(0, 100, size=10000),
        "c": randint(0, 100, size=10000),
    }
)

pandarallel.initialize(progress_bar=True)
df.parallel_apply(lambda x: x**2)
INFO: Pandarallel will run on 8 workers.
INFO: Pandarallel will use standard multiprocessing data transfer (pipe) to transfer data between the main process and workers.
a b c
0 3025 324 441
1 1 6561 5329
2 2025 4900 1024
3 25 5776 25
4 16 8100 3364
... ... ... ...
9995 49 676 4761
9996 3721 6889 4
9997 4225 9025 1156
9998 361 9 529
9999 5041 25 81

10000 rows Γ— 3 columns

Link to pandarallel.

6.12.3. PandasAI: Gain Insights From Your pandas DataFrame With AI#

Hide code cell content
!pip install pandasai

If you want to quickly gain insights from your pandas DataFrame with AI, use PandasAI. PandasAI serves as:

  • A tool to analyze your DataFrame

  • Not a tool to process your DataFrame

import pandas as pd  

df = pd.read_csv("https://raw.githubusercontent.com/mwaskom/seaborn-data/master/flights.csv")
df.head(10)
year month passengers
0 1949 January 112
1 1949 February 118
2 1949 March 132
3 1949 April 129
4 1949 May 121
5 1949 June 135
6 1949 July 148
7 1949 August 148
8 1949 September 136
9 1949 October 119
print(df.head(5).to_markdown())
|    | year                | month    |   passengers |
|---:|:--------------------|:---------|-------------:|
|  0 | 1949-01-01 00:00:00 | January  |          112 |
|  1 | 1949-01-01 00:00:00 | February |          118 |
|  2 | 1949-01-01 00:00:00 | March    |          132 |
|  3 | 1949-01-01 00:00:00 | April    |          129 |
|  4 | 1949-01-01 00:00:00 | May      |          121 |
from pandasai import PandasAI
from pandasai.llm.openai import OpenAI

# Instantiate a LLM
llm = OpenAI(api_token="YOUR_API_TOKEN")

# Use pandasai
pandas_ai = PandasAI(llm, conversational=False)
print(
    pandas_ai.run(
        df,
        prompt="Which month of the years has the highest number of passengers on average?",
    )
)
The month with the highest average number of passengers is: July
print(
    pandas_ai.run(
        df, prompt="Which are the five years with the highest passenger numbers?"
    )
)
year
1960    5714
1959    5140
1958    4572
1957    4421
1956    3939
Name: passengers, dtype: int64
print(pandas_ai.run(df, prompt="Within what range of years does the dataset span?"))
        year     month  passengers
0 1949-01-01   January         112
1 1949-01-01  February         118
2 1949-01-01     March         132
3 1949-01-01     April         129
4 1949-01-01       May         121
The dataset spans from 1949 to 1960.

Link to PandasAI.

6.12.4. fugue: Use pandas Functions on the Spark and Dask Engines.#

Hide code cell content
!pip install fugue pyspark

Wouldn’t it be nice if you can leverage Spark or Dask to parallelize data science workloads using pandas syntax? Fugue allows you to do exactly that.

Fugue provides the transform function allowing users to use pandas functions on the Spark and Dask engines.

import pandas as pd
from typing import Dict
from fugue import transform
from fugue_spark import SparkExecutionEngine

input_df = pd.DataFrame({"id": [0, 1, 2], "fruit": (["apple", "banana", "orange"])})
map_price = {"apple": 2, "banana": 1, "orange": 3}


def map_price_to_fruit(df: pd.DataFrame, mapping: dict) -> pd.DataFrame:
    df["price"] = df["fruit"].map(mapping)
    return df


df = transform(
    input_df,
    map_price_to_fruit,
    schema="*, price:int",
    params=dict(mapping=map_price),
    engine=SparkExecutionEngine,
)
df.show()
21/10/01 11:17:05 WARN Utils: Your hostname, khuyen-Precision-7740 resolves to a loopback address: 127.0.1.1; using 192.168.1.90 instead (on interface wlp111s0)
21/10/01 11:17:05 WARN Utils: Set SPARK_LOCAL_IP if you need to bind to another address
WARNING: An illegal reflective access operation has occurred
WARNING: Illegal reflective access by org.apache.spark.unsafe.Platform (file:/home/khuyen/book/venv/lib/python3.8/site-packages/pyspark/jars/spark-unsafe_2.12-3.1.2.jar) to constructor java.nio.DirectByteBuffer(long,int)
WARNING: Please consider reporting this to the maintainers of org.apache.spark.unsafe.Platform
WARNING: Use --illegal-access=warn to enable warnings of further illegal reflective access operations
WARNING: All illegal access operations will be denied in a future release
21/10/01 11:17:05 WARN NativeCodeLoader: Unable to load native-hadoop library for your platform... using builtin-java classes where applicable
Using Spark's default log4j profile: org/apache/spark/log4j-defaults.properties
Setting default log level to "WARN".
To adjust logging level use sc.setLogLevel(newLevel). For SparkR, use setLogLevel(newLevel).
21/10/01 11:17:06 WARN Utils: Service 'SparkUI' could not bind on port 4040. Attempting port 4041.
[Stage 2:===============>                                          (3 + 8) / 11]
+---+------+-----+
| id| fruit|price|
+---+------+-----+
|  0| apple|    2|
|  1|banana|    1|
|  2|orange|    3|
+---+------+-----+
[Stage 2:==========================================>               (8 + 3) / 11]

                                                                                

Link to fugue.

6.12.5. Version Your Pandas DataFrame with Delta Lake#

Hide code cell content
!pip install deltalake

Versioning your data is essential to undoing mistakes, preventing data loss, and ensuring reproducibility. Delta Lake makes it easy to version pandas DataFrames and review past changes for auditing and debugging purposes.

To version a pandas DataFrame with Delta Lake, start with writing out a pandas DataFrame to a Delta table.

import pandas as pd
import os
from deltalake.writer import write_deltalake

df = pd.DataFrame({"x": [1, 2, 3]})

# Write to a delta table 
table = "delta_lake"
os.makedirs(table, exist_ok=True)
write_deltalake(table, df)

Delta Lake stores the data in a Parquet file and maintains a transaction log that records the data operations, enabling time travel and versioning.

delta_lake:

 β”œβ”€β”€  0-4719861e-1d3a-49f8-8870-225e4e46e3a0-0.parquet  
 └──  _delta_log/ 
 β”‚  └────  00000000000000000000.json  

To load the Delta table as a pandas DataFrame, simply use the DeltaTable object:

from deltalake import DeltaTable

dt = DeltaTable(table)
dt.to_pandas()
x
0 1
1 2
2 3

Let’s see what happens when we append another pandas DataFrame to the Delta table.

df2 = pd.DataFrame({"x": [8, 9, 10]})

write_deltalake(table, df2, mode="append")
# Create delta table
dt = DeltaTable(table)
print(dt.to_pandas().to_markdown())
|    |   x |
|---:|----:|
|  0 |   1 |
|  1 |   2 |
|  2 |   3 |
|  3 |   8 |
|  4 |   9 |
|  5 |  10 |

Our Delta table now has two versions. Version 0 contains the initial data and Version 1 includes the data that was appended.

To get the metadata of files that currently make up the current table such as creation time, size, and statistics, call the get_add_actions method.

print(dt.get_add_actions(flatten=True).to_pandas())
                                               path  size_bytes   
0  0-a6738752-efca-4577-8cbf-9c69b404f2ee-0.parquet        1654  \
1  1-7a6df896-715a-4d4a-b210-b12e3fe57bc6-0.parquet        1654   

        modification_time  data_change  num_records  null_count.x  min.x   
0 2023-06-21 14:07:14.479         True            3             0      1  \
1 2023-06-21 14:07:18.657         True            3             0      8   

   max.x  
0      3  
1     10  

To access prior versions, simply specify the version number when loading the Delta table:

# Read Version 0 of the dataset
dt0 = DeltaTable(table, version=0)
dt0.to_pandas()
x
0 1
1 2
2 3

Link to delta-rs.

6.12.6. Optimize Query Speed with Data Partitioning#

Partitioning data allows queries to target specific segments rather than scanning the entire table, which speeds up data retrieval.

The following code uses Delta Lake to select partitions from a pandas DataFrame. Partitioned data loading is approximately 24.5 times faster than loading the complete dataset and then querying a particular subset

import pandas as pd
from deltalake.writer import write_deltalake
from deltalake import DeltaTable
from datetime import datetime
import numpy as np
# Create a DataFrame with hourly sales data for 2 million records
np.random.seed(0)  # For reproducibility

start_date = datetime(2023, 1, 1)
end_date = datetime(2023, 8, 31)
date_range = pd.date_range(start_date, end_date, freq='H')

data = {
    'datetime': date_range,
    'value': np.random.randint(100, 1000, len(date_range))
}

df = pd.DataFrame(data)
df['month'] = df['datetime'].dt.month
df['day'] = df['datetime'].dt.day
df['hour'] = df['datetime'].dt.hour
df[["month", "day", "hour", "value"]].head(5)
month day hour value
0 1 1 0 784
1 1 1 1 659
2 1 1 2 729
3 1 1 3 292
4 1 1 4 935
# Write to a Delta table
table_path = 'delta_lake'
write_deltalake(table_path, df)
%%timeit
# Load the data from the Delta table
DeltaTable(table_path).to_pandas().query("month == 1 & day == 1")
79.2 ms Β± 2.62 ms per loop (mean Β± std. dev. of 7 runs, 1 loop each)
# Write to a Delta table
table_path = "delta_lake2"
write_deltalake(table_path, df, partition_by=["month", "day"])
%%timeit
# Load the data from the Delta table
DeltaTable(table_path).to_pandas([("month", "=", "1"), ("day", "=", "1")])
3.23 ms Β± 181 Β΅s per loop (mean Β± std. dev. of 7 runs, 100 loops each)

Link to delta-rs.

6.12.7. Overwrite Partitions of a pandas DataFrame#

Hide code cell content
!pip install deltalake

If you need to modify a specific subset of your pandas DataFrame, such as yesterday’s data, it is not possible to overwrite only that partition. Instead, you have to load the entire DataFrame into memory as a workaround solution.

Delta Lake makes it easy to overwrite partitions of a pandas DataFrame.

First, write out a pandas DataFrame as a Delta table that is partitioned by the date column.

import pandas as pd
from deltalake.writer import write_deltalake
from deltalake import DeltaTable
table_path = "tmp/records" 
df = pd.DataFrame(
    {"a": [1, 2, 3], "date": ["04-21", "04-22", "04-22"]}
)
write_deltalake(
    table_path,
    df,
    partition_by=["date"],
)

The Delta table’s contents are partitioned by date, with each partition represented by a directory

 └──  _delta_log/ 
 β”‚  └────  00000000000000000000.json  
 └──  date=04-21/ 
 β”‚  └────  0-a6813d0c-157b-4ca6-8b3c-8d5afd51947c-0.parquet  
 └──  date=04-22/ 
 β”‚  └────  0-a6813d0c-157b-4ca6-8b3c-8d5afd51947c-0.parquet  

View the Delta table as a pandas DataFrame:

DeltaTable(table_path).to_pandas()
a date
0 2 04-22
1 3 04-22
2 1 04-21

Next, create another DataFrame with two other records on 04-22. Overwrite the 04-22 partition with the new DataFrame and leave other partitions untouched.

df = pd.DataFrame(
    {"a": [7, 8], "date": ["04-22", "04-22"]}
)
write_deltalake(
    table_path,
    df,
    mode="overwrite",
    partition_filters=[("date", "=", "04-22")],
)
DeltaTable(table_path).to_pandas()
a date
0 1 04-21
1 7 04-22
2 8 04-22

Here is the updated contents of the Delta table:

 └──  _delta_log/ 
 β”‚  └────  00000000000000000000.json
 β”‚  └────  00000000000000000001.json    
 └──  date=04-21/ 
 β”‚  └────  0-a6813d0c-157b-4ca6-8b3c-8d5afd51947c-0.parquet  
 └──  date=04-22/ 
 β”‚  β”œβ”€β”€β”€β”€  0-a6813d0c-157b-4ca6-8b3c-8d5afd51947c-0.parquet  
 β”‚  └────  1-b5c9640f-f386-4754-b28f-90e361ab4320-0.parquet 

Since the data files are not physically removed from disk, you can time travel to the initial version of the data.

DeltaTable(table_path, version=0).to_pandas()
a date
0 2 04-22
1 3 04-22
2 1 04-21

Link to delta-rs.

6.12.8. Efficient Data Appending in Parquet Files: Delta Lake vs. Pandas#

Hide code cell content
!pip install deltalake

Appending data to an existing Parquet file using pandas involves:

  • Loading the entire existing table into memory.

  • Merging the new data with the existing table.

  • Writing the merged data to the existing file.

This process can be time-consuming and memory-intensive.

import pandas as pd  

df1 = pd.DataFrame([
    (1, "John", 5000),
    (2, "Jane", 6000),
], columns=["employee_id", "employee_name", "salary"])

df2 = pd.DataFrame([
    (3, "Alex", 8000),
], columns=["employee_id", "employee_name", "salary"])
# Save to a parquet file
df1.to_parquet("data.parquet")

# Read the data
existing_data = pd.read_parquet("data.parquet")

# Concat two dataframes
df3 = pd.concat([df1, df2])

# Save to a file
df3.to_parquet("data.parquet")

Delta Lake offers a more efficient approach to handling this process. With Delta Lake, you can add, remove, or modify columns without the need to recreate the entire table.

Delta Lake is also built on top of the Parquet file format so it retains the efficiency and columnar storage benefits of Parquet.

from deltalake.writer import write_deltalake

table_path = "employees"

# Write to Delta Lake
write_deltalake(table_path, df1)

# Append to Delta Lake
write_deltalake(table_path, df2, mode="append")

Link to delta-rs.

6.12.9. Enforce Data Quality with Delta Lake Constraints#

Delta Lake provides a convenient way to enforce data quality by adding constraints to a table, ensuring that only valid and consistent data can be added.

In the provided code, attempting to add new data with a negative salary violates the constraint of a positive salary, and thus, the data is not added to the table.

Hide code cell content
!pip install deltalake
import pandas as pd
from deltalake.writer import write_deltalake
from deltalake import DeltaTable
table_path = "delta_lake"
df1 = pd.DataFrame(
    [
        (1, "John", 5000),
        (2, "Jane", 6000),
    ],
    columns=["employee_id", "employee_name", "salary"],
)

write_deltalake(table_path, df1)
print(df1.to_markdown())
|    |   employee_id | employee_name   |   salary |
|---:|--------------:|:----------------|---------:|
|  0 |             1 | John            |     5000 |
|  1 |             2 | Jane            |     6000 |
table = DeltaTable(table_path)
table.alter.add_constraint({"salary_gt_0": "salary > 0"})
df2 = pd.DataFrame(
    [(3, "Alex", -200)],
    columns=["employee_id", "employee_name", "salary"],
)

write_deltalake(table, df2, mode="append", engine="rust")
DeltaProtocolError: Invariant violations: ["Check or Invariant (salary > 0) violated by value in row: [3, Alex, -200]"]

Link to delta-rs.

6.12.10. Efficient Data Updates and Scanning with Delta Lake#

Hide code cell content
!pip install -U "deltalake==0.10.1"

Every time new data is appended to an existing Delta table, a new Parquet file is generated. This allows data to be ingested incrementally without having to rewrite the entire dataset.

As files accumulate, read operations may surge. The compact function merges small files into larger ones, enhancing scanning performance.

Combining incremental processing with the compact function enables efficient data updates and scans as your dataset expands.

import pandas as pd
from deltalake.writer import write_deltalake

table_path = 'delta_lake'
data_url = "https://gist.githubusercontent.com/khuyentran1401/458905fc5c630d7a1f7a510a04e5e0f9/raw/5b2d760011c9255a68eb08b83b3b8759ffa25d5c/data.csv"
dfs = pd.read_csv(data_url, chunksize=100)

for df in dfs:
    write_deltalake(table_path, df, mode="append")
from deltalake import DeltaTable

dt = DeltaTable(table_path)
%%timeit
df = dt.to_pandas()
30.6 ms Β± 2.94 ms per loop (mean Β± std. dev. of 7 runs, 10 loops each)
dt.optimize.compact()
{'numFilesAdded': 1,
 'numFilesRemoved': 100,
 'filesAdded': {'min': 278115,
  'max': 278115,
  'avg': 278115.0,
  'totalFiles': 1,
  'totalSize': 278115},
 'filesRemoved': {'min': 5712,
  'max': 5717,
  'avg': 5715.8,
  'totalFiles': 100,
  'totalSize': 571580},
 'partitionsOptimized': 1,
 'numBatches': 100,
 'totalConsideredFiles': 100,
 'totalFilesSkipped': 0,
 'preserveInsertionOrder': True}
%%timeit
df = dt.to_pandas()
1.32 ms Β± 49 Β΅s per loop (mean Β± std. dev. of 7 runs, 1,000 loops each)

Link to delta-rs.

6.12.11. Simplify Table Merge Operations with Delta Lake#

Hide code cell content
!pip install delta-spark

Merging two datasets and performing both insert and update operations can be a complex task.

Delta Lake makes it easy to perform multiple data manipulation operations during a merge operation.

The following code demonstrates merging two datasets using Delta Lake:

  • If a match is found, the last_talk column in people_table is updated with the corresponding value from new_df.

  • If the last_talk value in people_table is older than 30 days and the corresponding row is not present in the new_df table, the status column is updated to β€˜rejected’.

Hide code cell content
import pyspark
from delta import *

# Configure Spark to use Delta
builder = (
    pyspark.sql.SparkSession.builder.appName("MyApp")
    .config("spark.sql.extensions", "io.delta.sql.DeltaSparkSessionExtension")
    .config(
        "spark.sql.catalog.spark_catalog",
        "org.apache.spark.sql.delta.catalog.DeltaCatalog",
    )
)

spark = configure_spark_with_delta_pip(builder).getOrCreate()
Hide code cell content
# Create a spark dataframe
data = [
    (0, "A", "2023-04-15", "interviewing"),
    (1, "B", "2023-05-01", "interviewing"),
    (2, "C", "2023-03-01", "interviewing"),

]

df = (
    spark.createDataFrame(data)
    .toDF("id", "company", "last_talk", "status")
    .repartition(1)
)

# Write to a delta table
path = "tmp/interviews"
df.write.format("delta").save(path)
Hide code cell content
from delta.tables import DeltaTable

# Update the delta table
people_table = DeltaTable.forPath(spark, path)
# Target table
people_table.toDF().show()
+---+-------+----------+------------+
| id|company| last_talk|      status|
+---+-------+----------+------------+
|  0|      A|2023-05-07|interviewing|
|  1|      B|2023-05-01|interviewing|
|  2|      C|2023-03-01|    rejected|
+---+-------+----------+------------+
Hide code cell content
new_data = [(0, "A", "2023-05-07")]
new_df = (
    spark.createDataFrame(new_data).toDF("id", "company", "last_talk").repartition(1)
)
# Source table
new_df.show()
+---+-------+----------+
| id|company| last_talk|
+---+-------+----------+
|  0|      A|2023-05-07|
+---+-------+----------+
one_month_ago = "current_date() - INTERVAL '30' DAY"

people_table.alias("target").merge(
    new_df.alias("source"), "target.id = source.id"
).whenMatchedUpdate(
    set={"target.last_talk": "source.last_talk", "target.status": "'interviewing'"}
).whenNotMatchedBySourceUpdate(
    condition=f"target.last_talk <= {one_month_ago}",
    set={"target.status": "'rejected'"},
).execute()
people_table.toDF().show()
+---+-------+----------+------------+
| id|company| last_talk|      status|
+---+-------+----------+------------+
|  0|      A|2023-05-07|interviewing|
|  1|      B|2023-05-01|interviewing|
|  2|      C|2023-03-01|    rejected|
+---+-------+----------+------------+

Link to Delta Lake.

6.12.12. The Best Way to Append Mismatched Data to Parquet Tables#

Appending mismatched data to a Parquet table involves reading the existing data, concatenating it with the new data, and overwriting the existing Parquet file. This approach can be expensive and may lead to schema inconsistencies.

In the following code, the datatype of col3 is supposed to be int64 instead of float64.

import pandas as pd  

filepath = 'test.parquet'

# Write a dataframe to a parquet file
df1 = pd.DataFrame({'col1': [1, 2], 'col2': [3, 4]})
df1.to_parquet(filepath)

# Append a dataframe to a parquet file
df2 = pd.DataFrame({'col1': [2], 'col2': [7], 'col3': [0]})
concatenation = pd.concat([df1, df2]) # concatenate dataframes
concatenation.to_parquet(filepath) # overwrite original file
concat_df = pd.read_parquet(filepath)
print(concat_df, "\n")
print(concat_df.dtypes)
   col1  col2  col3
0     1     3   NaN
1     2     4   NaN
0     2     7   0.0 

col1      int64
col2      int64
col3    float64
dtype: object

With Delta Lake, you can effortlessly append DataFrames with extra columns while ensuring the preservation of your data’s schema.

Hide code cell content
import pyspark
from delta import *

# Configure Spark to use Delta
builder = (
    pyspark.sql.SparkSession.builder.appName("MyApp")
    .config("spark.sql.extensions", "io.delta.sql.DeltaSparkSessionExtension")
    .config(
        "spark.sql.catalog.spark_catalog",
        "org.apache.spark.sql.delta.catalog.DeltaCatalog",
    )
)

spark = configure_spark_with_delta_pip(builder).getOrCreate()
# Create a spark Dataframe
data = [(1, 3), (2, 4)]

df1 = (
    spark.createDataFrame(data)
    .toDF("col1", "col2")
    .repartition(1)
)

# Write to a delta table
path = "tmp"
df1.write.format("delta").save(path)
                                                                                
# Create a new DataFrame
new_data = [(2, 7, 0)]
df2 = (
    spark.createDataFrame(new_data).toDF("col1", "col2", "col3").repartition(1)
)
df2.show()
+----+----+----+
|col1|col2|col3|
+----+----+----+
|   2|   7|   0|
+----+----+----+
# Append to the existing Delta table
df2.write.option("mergeSchema", "true").mode("append").format("delta").save(path)
                                                                                
# Read the Delta table
from delta.tables import DeltaTable

table = DeltaTable.forPath(spark, path)
concat_df = table.toDF().pandas_api()

print(concat_df, "\n")
print(concat_df.dtypes)
   col1  col2  col3
0     2     7   0.0
1     1     3   NaN
2     2     4   NaN 

col1    int64
col2    int64
col3    int64
dtype: object

Link to Delta Lake.

6.12.13. Polars: Blazing Fast DataFrame Library#

Hide code cell content
!pip install polars

If you want data manipulation library that’s both fast and memory-efficient, try Polars. Polars provides a high-level API similar to Pandas but with better performance for large datasets.

The code below compares the performance of Polars and pandas.

import pandas as pd
import polars as pl
import numpy as np
import time

# Create two Pandas DataFrames with 1 million rows each
pandas_df1 = pd.DataFrame({
    'key': np.random.randint(0, 1000, size=1_000_000),
    'value1': np.random.rand(1_000_000)
})

pandas_df2 = pd.DataFrame({
    'key': np.random.randint(0, 1000, size=1_000_000),
    'value2': np.random.rand(1000000)
})

# Create two Polars DataFrames from the Pandas DataFrames
polars_df1 = pl.from_pandas(pandas_df1)
polars_df2 = pl.from_pandas(pandas_df2)

# Merge the two DataFrames on the 'key' column
start_time = time.time()
pandas_merged = pd.merge(pandas_df1, pandas_df2, on='key')
pandas_time = time.time() - start_time

start_time = time.time()
polars_merged = polars_df1.join(polars_df2, on='key')
polars_time = time.time() - start_time

print(f"Pandas time: {pandas_time:.6f} seconds")
print(f"Polars time: {polars_time:.6f} seconds")
Pandas time: 127.604390 seconds
Polars time: 41.079080 seconds
print(f"Polars is {pandas_time/polars_time:.2f} times faster than Pandas")
Polars is 3.11 times faster than Pandas

Link to polars

6.12.14. Polars: Speed Up Data Processing 12x with Lazy Execution#

Hide code cell content
!pip install polars

Polars is a lightning-fast DataFrame library that utilizes all available cores on your machine.

Polars has two APIs: an eager API and a lazy API.

The eager execution is similar to Pandas, which executes code immediately.

In contrast, the lazy execution defers computations until the collect() method is called. This approach avoids unnecessary computations, making lazy execution potentially more efficient than eager execution.

The code following code shows filter operations on a DataFrame containing 10 million rows. Running polars with lazy execution is 12 times faster than using pandas.

Hide code cell content
import numpy as np

# Create a random seed for reproducibility
np.random.seed(42)

# Number of rows in the dataset
num_rows = 10_000_000

# Sample data for categorical columns
categories = ["a", "b", "c", "d"]

# Generate random data for the dataset
data = {
    "Cat1": np.random.choice(categories, size=num_rows),
    "Cat2": np.random.choice(categories, size=num_rows),
    "Num1": np.random.randint(1, 100, size=num_rows),
    "Num2": np.random.randint(1000, 10000, size=num_rows),
}

Create a pandas DataFrame and filter the DataFrame.

import pandas as pd


df = pd.DataFrame(data)
df.head()
Cat1 Cat2 Num1 Num2
0 c a 40 7292
1 d b 45 7849
2 a a 93 6940
3 c a 46 1265
4 c a 98 2509
%timeit df[(df['Cat1'] == 'a') & (df['Cat2'] == 'b') & (df['Num1'] >= 70)]
706 ms Β± 75.4 ms per loop (mean Β± std. dev. of 7 runs, 1 loop each)

Create a polars DataFrame and filter the DataFrame.

import polars as pl

pl_df = pl.DataFrame(data)
%timeit pl_df.lazy().filter((pl.col('Cat1') == 'a') & (pl.col('Cat2') == 'b') & (pl.col('Num1') >= 70)).collect()
58.1 ms Β± 428 Β΅s per loop (mean Β± std. dev. of 7 runs, 10 loops each)

Link to polars

6.12.15. Polars vs. Pandas for CSV Loading and Filtering#

Hide code cell content
!pip install polars
Hide code cell content
!wget -O airport-codes.csv "https://datahub.io/core/airport-codes/r/0.csv"

The read_csv method in Pandas loads all rows of the dataset into the DataFrame before filtering to remove all unwanted rows.

On the other hand, the scan_csv method in Polars delays execution and optimizes the operation until the collect method is called. This approach accelerates code execution, particularly when handling large datasets.

In the code below, it is 25.5 times faster to use Polars instead of Pandas to read a subset of CSV file containing 57k rows.

import pandas as pd
import polars as pl 
%%timeit
df = pd.read_csv("airport-codes.csv")
df[(df["type"] == "heliport") & (df["continent"] == "EU")]
143 ms Β± 8.3 ms per loop (mean Β± std. dev. of 7 runs, 10 loops each)
%%timeit
pl.scan_csv("airport-codes.csv").filter(
    (pl.col("type") == "heliport") & (pl.col("continent") == "EU")
).collect()
5.6 ms Β± 594 Β΅s per loop (mean Β± std. dev. of 7 runs, 100 loops each)

6.12.16. Simple and Expressive Data Transformation with Polars#

Hide code cell content
!pip install polars

Compared to pandas, Polars provides a more expressive syntax for creating complex data transformation pipelines. Every expression in Polars produces a new expression, and these expressions can be piped together.

import pandas as pd

df = pd.DataFrame(
    {"A": [1, 2, 6], "B": ["a", "b", "c"], "C": [True, False, True]}
)
integer_columns = df.select_dtypes("int64")
other_columns = df[["B"]]
pd.concat([integer_columns, other_columns], axis=1)
A B
0 1 a
1 2 b
2 6 c
import polars as pl

pl_df = pl.DataFrame(
    {"A": [1, 2, 6], "B": ["a", "b", "c"], "C": [True, False, True]}
)
pl_df.select([pl.col(pl.Int64), "B"])
shape: (3, 2)
AB
i64str
1"a"
2"b"
6"c"

6.12.17. Harness Polars and Delta Lake for Blazing Fast Performance#

Hide code cell content
!pip install polars deltalake

Polars is a DataFrame library written in Rust that has blazing fast performance. Delta Lake has helpful features including ACID transactions, time travel, schema enforcement, and more. Combining these two tools makes the code exceptionally powerful and efficient for data processing and analysis.

In the code below, using Polars for grouping operations is 6.88 times faster than using Pandas. Moreover, the integration of Delta Lake enables seamless time travel across different versions of the Polars DataFrame.

import pandas as pd
import numpy as np

# Number of rows in the dataset
num_rows = 10_000_000

# Creating categorical columns
category_col1 = np.random.choice(['A', 'B', 'C'], size=num_rows)

# Creating numerical columns
numeric_col1 = np.random.randint(low=1, high=100, size=num_rows)

# Creating the DataFrame
data = {
    'Cat1': category_col1,
    'Num1': numeric_col1,
}

df = pd.DataFrame(data)

# Outputting the first few rows of the dataset
df.tail()
Cat1 Num1
9999995 B 42
9999996 A 23
9999997 A 79
9999998 B 87
9999999 B 19
pandas_time = %timeit -o df.groupby('Cat1').sum()
277 ms Β± 55.7 ms per loop (mean Β± std. dev. of 7 runs, 1 loop each)

Create two versions of the Delta table:

from deltalake.writer import write_deltalake

save_path = "tmp/bear_delta_lake"

write_deltalake(save_path, df)
df2 = pd.DataFrame({"Cat1": ["B", "C"], "Num1": [2, 3]})

write_deltalake(save_path, df2, mode="append")

Read the latest version of the Delta table with Polars:

import polars as pl 

pl_df = pl.read_delta(save_path)
print(pl_df.tail())
shape: (5, 2)
β”Œβ”€β”€β”€β”€β”€β”€β”¬β”€β”€β”€β”€β”€β”€β”
β”‚ Cat1 ┆ Num1 β”‚
β”‚ ---  ┆ ---  β”‚
β”‚ str  ┆ i64  β”‚
β•žβ•β•β•β•β•β•β•ͺ══════║
β”‚ A    ┆ 43   β”‚
β”‚ A    ┆ 14   β”‚
β”‚ C    ┆ 3    β”‚
β”‚ B    ┆ 2    β”‚
β”‚ C    ┆ 3    β”‚
β””β”€β”€β”€β”€β”€β”€β”΄β”€β”€β”€β”€β”€β”€β”˜
polars_time = %timeit -o pl_df.groupby("Cat1").sum()
40.3 ms Β± 3.22 ms per loop (mean Β± std. dev. of 7 runs, 10 loops each)

Compare the performance of grouping operations using Pandas and Polars:

round(pandas_time.average/polars_time.average, 3)
6.882
print(f"Grouping operations using Polars is {pandas_time.average/polars_time.average:.2f} times faster than using Pandas.")
Grouping operations using Polars is 6.88 times faster than using Pandas.

Time travel to the version 0 of Delta table

print(pl.read_delta(save_path, version=0).tail())
shape: (5, 2)
β”Œβ”€β”€β”€β”€β”€β”€β”¬β”€β”€β”€β”€β”€β”€β”
β”‚ Cat1 ┆ Num1 β”‚
β”‚ ---  ┆ ---  β”‚
β”‚ str  ┆ i64  β”‚
β•žβ•β•β•β•β•β•β•ͺ══════║
β”‚ C    ┆ 95   β”‚
β”‚ B    ┆ 38   β”‚
β”‚ A    ┆ 43   β”‚
β”‚ A    ┆ 14   β”‚
β”‚ C    ┆ 3    β”‚
β””β”€β”€β”€β”€β”€β”€β”΄β”€β”€β”€β”€β”€β”€β”˜

Link to polars

Link to delta-rs.

6.12.18. Parallel Execution of Multiple Files with Polars#

Hide code cell content
!pip install polars

If you have multiple files to process, Polars enables you to construct a query plan for each file beforehand. This allows for the efficient execution of multiple files concurrently, maximizing processing speed.

import glob

import polars as pl

# Construct a query plan for each file
queries = []
for file in glob.glob("test_data/*.csv"):
    q = pl.scan_csv(file).group_by("Cat").agg(pl.sum("Num"))
    queries.append(q)

# Execute files in parallel
dataframes = pl.collect_all(queries)
dataframes
[shape: (3, 2)
 β”Œβ”€β”€β”€β”€β”€β”¬β”€β”€β”€β”€β”€β”
 β”‚ Cat ┆ Num β”‚
 β”‚ --- ┆ --- β”‚
 β”‚ str ┆ i64 β”‚
 β•žβ•β•β•β•β•β•ͺ═════║
 β”‚ A   ┆ 2   β”‚
 β”‚ C   ┆ 6   β”‚
 β”‚ B   ┆ 4   β”‚
 β””β”€β”€β”€β”€β”€β”΄β”€β”€β”€β”€β”€β”˜,
 shape: (3, 2)
 β”Œβ”€β”€β”€β”€β”€β”¬β”€β”€β”€β”€β”€β”
 β”‚ Cat ┆ Num β”‚
 β”‚ --- ┆ --- β”‚
 β”‚ str ┆ i64 β”‚
 β•žβ•β•β•β•β•β•ͺ═════║
 β”‚ B   ┆ 5   β”‚
 β”‚ A   ┆ 1   β”‚
 β”‚ C   ┆ 1   β”‚
 β””β”€β”€β”€β”€β”€β”΄β”€β”€β”€β”€β”€β”˜,
 shape: (3, 2)
 β”Œβ”€β”€β”€β”€β”€β”¬β”€β”€β”€β”€β”€β”
 β”‚ Cat ┆ Num β”‚
 β”‚ --- ┆ --- β”‚
 β”‚ str ┆ i64 β”‚
 β•žβ•β•β•β•β•β•ͺ═════║
 β”‚ C   ┆ 4   β”‚
 β”‚ A   ┆ 4   β”‚
 β”‚ B   ┆ 1   β”‚
 β””β”€β”€β”€β”€β”€β”΄β”€β”€β”€β”€β”€β”˜]

Link to polars

6.12.19. Polars’ Streaming Mode: A Solution for Large Data Sets#

Hide code cell content
!pip install polars

The default collect method in Polars processes your data as a single batch, which means that all the data must fit into your available memory.

If your data requires more memory than you have available, Polars can process it in batches using streaming mode. To use streaming mode, simply pass the streaming=True argument to the collect method.

import polars as pl

df = (
    pl.scan_csv("reddit.csv")
    .with_columns(pl.col("name").str.to_uppercase())
    .filter(pl.col("comment_karma") > 0)
    .collect(streaming=True)
)

Learn more about Streaming API in Polars.