Writing Data Out

Saving a result, and the index that gets written with it.

to_csv writes the index too

By default the index becomes an unnamed first column. Reading that file back gives you a stray "Unnamed: 0", which is where most mysterious extra columns come from.

import pandas as pd

df = pd.DataFrame({"name": ["Ada"], "score": [92]})
df.to_csv("out.csv", index=False)
print(open("out.csv").read())

Write what you meant to write

columns= picks what goes out, and index=False keeps the row numbers out of it unless they mean something.

import pandas as pd

df = pd.DataFrame({"name": ["Ada"], "score": [92], "note": ["x"]})
df.to_csv("out.csv", index=False, columns=["name", "score"])
print(open("out.csv").read().strip())

Exercise

Try It Yourself

Write save_scores(df, path) that writes the table to path as CSV with no index column, then returns the first line of the file.

import pandas as pd

def save_scores(df, path):
    # Write without the index, return the header line.
    return ""

print(save_scores(pd.DataFrame({"name": ["Ada"], "score": [92]}), "out.csv"))

Press Run to see output

Check Your Understanding

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