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I'm trying to append a pandas DataFrame (single column) to an existing CSV, much like this post, but it's not working! Instead my column is added at the bottom of the csv, and repeated over and over (rows in csv >> size of column). Here's my code:
with open(outputPath, "a") as resultsFile:
print len(scores)
scores.to_csv(resultsFile, header=False)
print resultsFile
Terminal output:4032
<open file '/Users/alavin/nta/NAB/results/numenta/artificialWithAnomaly/numenta_art_load_balancer_spikes.csv', mode 'a' at 0x1088686f0>
Thank you in advance!
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To add a dataframe row-wise to an existing CSV file, we can write the dataframe to the CSV file in append mode by the parameter a using the pandas to_csv() function.
Open your CSV file in append mode Create a file object for this file. Pass the file object and a list of column names to DictWriter() You will get an object of DictWriter. Pass the dictionary as an argument to the writerow() function of DictWriter (it will add a new row to the CSV file).
You can specify a python write mode in the pandas to_csv function. For append it is 'a'. The default mode is 'w'.
In pandas you can add/append a new column to the existing DataFrame using DataFrame. insert() method, this method updates the existing DataFrame with a new column. DataFrame. assign() is also used to insert a new column however, this method returns a new Dataframe after adding a new column.
Like what @aus_lacy has already suggested, you just need to read the csv file into a data frame first, concatenate two data frames and write it back to the csv file:
supposed your existing data frame called df:
df_csv = pd.read_csv(outputPath, 'your settings here')
# provided that their lengths match
df_csv['to new column'] = df['from single column']
df_csv.to_csv(outputPath, 'again your settings here')
That's it.
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