Writing A Pandas DataFrame To A Disk File Or A Buffer

Overview:

• Pandas DataFrame class supports storing data in two-dimensional format using nump.ndarray as the underlying data-structure.

• The DataFrame contents can be written to a disk file, to a text buffer through the method DataFrame.to_csv(), by passing the name of the CSV file or the text stream instance as a parameter.

To write the contents of a pandas DataFrame as a CSV file:

The Python example below writes the contents of a DataFrame which has volume data of three stocks for five trading days into a CSV file in the current working directory.

# Example Python program to write the contents of a
# DataFrame into a CSV file
import pandas as pds
 
# Name of the CSV file
csvFileName = "./Volumes.csv"
 
# Create a dictionary of lists for stock vs volume data
volumes = {"Stock1":[53871, 48421, 51761, 57987, 58900],
           "Stock2":[45012, 44521, 45569, 44732, 44721],
           "Stock3":[67540, 62890, 66743, 68900, 71456]};
 
# Load data into a DataFrame instance
dataFrame = pds.DataFrame(data=volumes);
dataFrame.index = ("Day 1", "Day 2", "Day 3", "Day 4", "Day 5");
 
# Write contents of the DataFrame to a CSV file
dataFrame.to_csv(csvFileName);

Output:

Contents of the CSV file Volumes.csv as written by the above example is given here.
,Stock1,Stock2,Stock3
Day 1,53871,45012,67540
Day 2,48421,44521,62890
Day 3,51761,45569,66743
Day 4,57987,44732,68900
Day 5,58900,44721,71456

To get the contents of a pandas DataFrame as a CSV String:

This Python example program creates a DataFrame from a Python dictionary and gets the DataFrame contents as a CSV string by calling DataFrame.to_csv() method, without passing any parameters.

# Example Python program to get the contents of a DataFrame as a CSV string

import pandas as pds

# Standard deviations in the thickness of three wooden board variants in mm
standardDeviations = {"Wood 1": [0.4, 0.5, 0.3],
                      "Wood 2": [0.1, 0.2, 0.3],
                      "Wood 3": [0.7, 0.6, 0.7]
                      };
          
# Load data into a DataFrame instance
dataFrame = pds.DataFrame(data=standardDeviations, index=("Variant1",
                                                          "Variant2",
                                                          "Variant3"));
 
# Get the contents of the DataFrame as a CSV file
csv=dataFrame.to_csv();
print("Contents of the DataFrame as a CSV string:")
print(csv);

Output:

Contents of the DataFrame as a CSV string:
,Wood 1,Wood 2,Wood 3
Variant1,0.4,0.1,0.7
Variant2,0.5,0.2,0.6
Variant3,0.3,0.3,0.7

To write a pandas DataFrame into a text buffer:

This Python example passes an instance of a text stream like StringIO() to write the DataFrame as a CSV into the in-memory text buffer.

# Example Python program to write the contents of a DataFrame to a buffer
import pandas as pds
from io import StringIO
 
# Closing price of 3 different stocks over 5 trading days
closingPrices = {"Stock1": [34.17, 34.25, 34.2, 34.24, 34.3],
                "Stock2": [10.01, 10.20, 10.1, 10.15, 10.2],
                "Stock3": [41.6, 42.1, 41.89, 42.4, 42.7]
               };
          
# Load data from the Python dictionary into a DataFrame instance
dataFrame = pds.DataFrame(data=closingPrices);
 
# Create an in-memory text stream
textStream = StringIO();
 
# Write the DataFrame contents to the text stream's buffer as a CSV
dataFrame.to_csv(textStream);
 
print("DataFrame as CSV (from the buffer):");
 
# Print the buffer contents
print(textStream.getvalue());

Output:

DataFrame as CSV (from the buffer):
,Stock1,Stock2,Stock3
0,34.17,10.01,41.6
1,34.25,10.2,42.1
2,34.2,10.1,41.89
3,34.24,10.15,42.4
4,34.3,10.2,42.7

Source: pythontic.com

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