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Version: Python

How to export data to CSV or other delimited files

This guide discusses how to export table data to CSV (or other delimited) files by using write_csv.

The basic syntax follows:

from deephaven import write_csv

write_csv(table, "/data/outputFile.csv")
note

Deephaven writes files to locations relative to the base of its Docker container. See Docker data volumes to learn more about the relation between locations in the container and the local file system.

We'll create a table to export by using empty_Table and update. The table contains 100 rows of trigonometric values.

from deephaven import empty_table

source = empty_table(100).update(formulas=[
    "X = 0.1 * i",
    "SinX = sin(X)",
    "CosX = cos(X)",
    "TanX = tan(X)"
])

Standard CSV files​

The simplest way to use write_csv is to supply two input parameters:

  • The Deephaven source table.
  • The path of the output CSV file.
from deephaven import write_csv

write_csv(source, "/data/TrigFunctions.csv")

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Null values​

Null values are common in tables. How are they handled when exporting data to a CSV? This depends on how you call write_csv.

First, let's create a table with null values. The example below uses a function to fill the SinX column with a large number of nulls.

from deephaven import empty_table

source_with_nulls = empty_table(100).update(formulas=[
    "X = 0.1 * i",
    "SinX = X % 0.2 < 0.01 ? NULL_DOUBLE : sin(X)",
    "CosX = cos(X)",
    "TanX = tan(X)"
])

The SinX column contains many null cells. The example below writes this table to a CSV file called TrigFunctionsWithNulls.csv.

from deephaven import write_csv

write_csv(source_with_nulls, "/data/TrigFunctionsWithNulls.csv")

img

Column selection​

In the event you don't want to write every column in the table to a CSV file, you can specify which columns to write. This is done by providing a list of column names, as shown below.

from deephaven import write_csv

write_csv(source_with_nulls, "/data/Cosine.csv", cols=["X", "CosX"])

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