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Load CSV Data in Metaflow Steps

Question​

I have a CSV and want to access it in a Metaflow flow. How can I read this data into tasks and write it to disk?

Solution​

You can read a CSV into the flow using IncludeFile. This makes the data accessible for local and remote steps in the flow.

1Acquire CSV​

In this example a CSV from the Metaflow tutorials is used. This is done in the save_data_locally function defined outside of the flow.

2Run Flow​

This flow shows how to:

  • Include a CSV saved locally for all steps in the flow.
  • Add a feature to each data point.
  • Save the new data as a flow artifact.
load_csv_data.py
from metaflow import FlowSpec, step, IncludeFile
import pandas as pd

def save_data_locally():
    url = "https://raw.githubusercontent.com/" + \
          "Netflix/metaflow/master/metaflow"
    data_path = "/tutorials/02-statistics/movies.csv"
    local_path = "./movies.csv"
    df = pd.read_csv(url+data_path)
    df.to_csv(local_path)

class CSVFlow(FlowSpec):
    
    data = IncludeFile("data", default="./movies.csv")
    
    @step
    def start(self):
        self.next(self.use_csv)
        
    @step
    def use_csv(self):
        import pandas as pd 
        from io import StringIO
        df = pd.read_csv(StringIO(self.data),
                         index_col=0)
        f = lambda x: x < 2000
        df["is_before_2000"] = df["title_year"].apply(f)
        self.new_df = df
        self.next(self.end)
        
    @step
    def end(self):
        result = self.new_df.is_before_2000.sum() 
        print(f"Number of pre-2000 movies is {result}.")
        
if __name__ == "__main__":
    save_data_locally()
    CSVFlow()
python load_csv_data.py run
    ...
     [1654221300950244/end/3 (pid 71595)] Task is starting.
     [1654221300950244/end/3 (pid 71595)] Number of pre-2000 movies is 1023.
     [1654221300950244/end/3 (pid 71595)] Task finished successfully.
    ...

3Access Artifacts Outside of Flow​

The following can be run in any script or notebook to access the contents of the DataFrame that was stored as a flow artifact with self.new_df. 

from metaflow import Flow 
run = Flow("CSVFlow").latest_run
assert run.successful
run.data.new_df.is_before_2000.sum()
    1023

Further Reading​