csv: Doesn't append a new row with writerow()

[LucasColas]

Bug report

When I want to add a new row to an existing file it doesn't append a new row. But it extends the last row.

def ajout_csv(nom, ordre, lignes):
  with open(nom + '.csv', mode='a', newline='') as csv_file:
      writer = csv.DictWriter(csv_file, fieldnames=ordre)
      #writer.writeheader()
      #writer = csv.writer(csv_file)
      for ligne in lignes:
        print("ligne",ligne)
        writer.writerow(ligne)
      csv_file.close()

ajout_csv('diabetes_UCI', 
          ['Pregnancies', 'Glucose', 'BloodPressure', 'SkinThickness', 'Insulin', 'BMI', 'DiabetesPedigreeFunction', 'Age', 'Outcome'],
          
          [{'Pregnancies': '42', 'Glucose': '73', 'BloodPressure': '50', 'SkinThickness': '10', 'Insulin': '0', 'BMI': '23', 'DiabetesPedigreeFunction': '0.248', 'Age': '21', 'Outcome': '0'}])

Results :

{'Pregnancies': '1', 'Glucose': '93', 'BloodPressure': '70', 'SkinThickness': '31', 'Insulin': '0', 'BMI': '30.4', 'DiabetesPedigreeFunction': '0.315', 'Age': '23', 'Outcome': '042', None: ['73', '50', '10', '0', '23', '0.248', '21', '0']}

[LucasColas]

Ok it seems it's the first append that doesn't append a new row. But if I do a second append it works.

[sobolevn]

A couple of comments:

1. I've executed your code, it works as expected. The first time I get one row 42,73,50,10,0,23,0.248,21,0, the second time I have two rows (they are obviously the same). Not sure what the bug is.
2. You don't need to csv_file.close(), it will be handled by with open() automatically. That the whole point of it

Going to mark this as pending to be closed, because of 1.

[akulakov]

It seems like Lucas is expecting that csv module will examine the file and if it's non-empty and doesn't end with a newline, prefix the row with a newline. That's not unreasonable but csv docs do not promise that, and it would be a backwards incompatible change, and it's very easy to add a newline explicitly if needed, so I agree this should be closed, and I don't think docs need to clarify this scenario.