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https://issues.apache.org/jira/browse/ARROW-10133?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17204605#comment-17204605
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Joris Van den Bossche commented on ARROW-10133:
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[~skyetetra] by default, pandas does currently not support missing values in
integer columns. So once you have integers with missing values, those will be
casted to floats. See
https://pandas.pydata.org/docs/user_guide/missing_data.html for more details.
So what you describe is expected behaviour. There is work under way to actually
support integers with missing values in pandas: de dtype itself will already
exist, and in the upcoming pandas 1.2 release there will also be an option to
use those dtypes when reading parquet files
(https://github.com/pandas-dev/pandas/pull/31242)
> [Python] parquet Int64 col cast to float64 on load in pandas
> ------------------------------------------------------------
>
> Key: ARROW-10133
> URL: https://issues.apache.org/jira/browse/ARROW-10133
> Project: Apache Arrow
> Issue Type: Bug
> Components: Python
> Affects Versions: 0.17.1
> Reporter: Jacqueline Nolis
> Priority: Minor
> Attachments: example-failed-int64.parquet
>
>
> Under certain conditions a saved parquet table with a column that is Int64
> and all NA seems to be cast to a float64 with all NaN on load. The desired
> behavior is to have it stay as Int64. Attached is a table where said issue
> occurs: the second column here should be a int64 but is being loaded as a
> float64 in Pandas.
>
> Interestingly, it seems to be correctly interpreting the column as a Int64
> when loading in R, so perhaps its only a Pandas issue.
>
> import pyarrow.parquet as pq
> import boto3
> import pandas as pd
> import io
> obj = boto3.client('s3').get_object(Bucket="...", Key='...') # file attached
> to ticket
> x = pq.read_table(io.BytesIO(obj['Body'].read()))
> y = x.to_pandas() # this is where the undesired int64 to a float64 cast occurs
> # >>> x
> # pyarrow.Table
> # product_id: string
> # cost: int64
> # name: string
> # >>> y.dtypes
> # product_id object
> # cost float64
> # name object
> # dtype: object
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