Pythonically add header to a csv file

Question

I wrote a Python script merging two csv files, and now I want to add a header to the final csv. I tried following the suggestions reported here and I got the following error: expected string, float found. What is the most pythonic way to fix this?

Here is the code I am using:

import csv

with open('combined_file.csv', 'w', newline='') as outcsv:
    writer = csv.DictWriter(outcsv, fieldnames = ["Date", "temperature 1", "Temperature 2"])
    writer.writeheader()

    with open('t1.csv', 'r', newline='') as incsv:
        reader = csv.reader(incsv)
        writer.writerows(row + [0.0] for row in reader)

    with open('t2.csv', 'r', newline='') as incsv:
        reader = csv.reader(incsv)
        writer.writerows(row[:1] + [0.0] + row[1:] for row in reader)

how many columns are you writing into your csv file? Could you please specify in your question 1. input format of your file 2. output format — nio
– nio, Commented Dec 3, 2013 at 10:09
@nio: A large section of the code posted is from this previous question by the OP — Martijn Pieters
– Martijn Pieters, Commented Dec 3, 2013 at 10:20

Martijn Pieters · Accepted Answer · 2019-07-10 16:26:58Z

172

The DictWriter() class expects dictionaries for each row. If all you wanted to do was write an initial header, use a regular csv.writer() and pass in a simple row for the header:

import csv

with open('combined_file.csv', 'w', newline='') as outcsv:
    writer = csv.writer(outcsv)
    writer.writerow(["Date", "temperature 1", "Temperature 2"])

    with open('t1.csv', 'r', newline='') as incsv:
        reader = csv.reader(incsv)
        writer.writerows(row + [0.0] for row in reader)

    with open('t2.csv', 'r', newline='') as incsv:
        reader = csv.reader(incsv)
        writer.writerows(row[:1] + [0.0] + row[1:] for row in reader)

The alternative would be to generate dictionaries when copying across your data:

import csv

with open('combined_file.csv', 'w', newline='') as outcsv:
    writer = csv.DictWriter(outcsv, fieldnames = ["Date", "temperature 1", "Temperature 2"])
    writer.writeheader()

    with open('t1.csv', 'r', newline='') as incsv:
        reader = csv.reader(incsv)
        writer.writerows({'Date': row[0], 'temperature 1': row[1], 'temperature 2': 0.0} for row in reader)

    with open('t2.csv', 'r', newline='') as incsv:
        reader = csv.reader(incsv)
        writer.writerows({'Date': row[0], 'temperature 1': 0.0, 'temperature 2': row[1]} for row in reader)

edited Jul 10, 2019 at 16:26

answered Dec 3, 2013 at 10:16

Martijn Pieters

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3 Comments

loretoparisi Over a year ago

This works, funny thing: when file is opened in a mode, writer.writeheader() will write down the header twice despite of the header row was written already!

Martijn Pieters Over a year ago

@loretoparisi: of course it does. Don't use writer.writeheader() when appending to an existing file. The csv.writer() object can't detect that you are writing data to an existing file.

flywire Over a year ago

Should check there are records to write before writing a header and only write header if at start of file.

Mitul Panchal · Accepted Answer · 2017-12-08 09:00:11Z

14

You just add one additional row before you execute the loop. This row contains your CSV file header name.

schema = ['a','b','c','b']
row = 4
generators = ['A','B','C','D']
with open('test.csv','wb') as csvfile:    
     writer = csv.writer(csvfile, delimiter=delimiter)
# Gives the header name row into csv
     writer.writerow([g for g in schema])   
#Data add in csv file       
     for x in xrange(rows):
         writer.writerow([g() for g in generators])

answered Dec 8, 2017 at 9:00

Mitul Panchal

6526 silver badges7 bronze badges

Comments

saggzz · Accepted Answer · 2018-06-13 13:37:27Z

7

This worked for me.

header = ['row1', 'row2', 'row3']
some_list = [1, 2, 3]
with open('test.csv', 'wt', newline ='') as file:
    writer = csv.writer(file, delimiter=',')
    writer.writerow(i for i in header)
    for j in some_list:
        writer.writerow(j)

edited Jun 13, 2018 at 13:37

answered Jun 13, 2018 at 12:45

saggzz

3113 silver badges9 bronze badges

2 Comments

Gorgonzola Over a year ago

using file as a variable is not a good idea. Line #3. Use csvfile or some other instead.

Nícolas Schirmer Over a year ago

@Gorgonzola It's just a syntax name in a StackOverflow answer... Could it be banana, but in this case it's just a file

Hietsh Kumar · Accepted Answer · 2021-05-18 14:02:12Z

2

Would not this be simple that if we are creating file that time only we add header if we are not than append data to it..

  conso_file_path = 'your_folder_path\your_file.csv'
    # Create File
    if not os.path.exists(conso_file_path):
        print("No File")
        header = ['col_name 1', 'col_name 1', 'col_name 3', 'col_name4']
        with open(conso_file_path, 'w') as f:
            writer = csv.writer(f)
            writer.writerow(header)

answered May 18, 2021 at 14:02

Hietsh Kumar

1,36911 silver badges18 bronze badges

Comments

G M · Accepted Answer · 2025-09-04 07:39:37Z

0

Since Python 3.8 there is a specific method reported int the documentation, here the official example:

import csv

with open('names.csv', 'w', newline='') as csvfile:
    fieldnames = ['first_name', 'last_name']
    writer = csv.DictWriter(csvfile, fieldnames=fieldnames)

    writer.writeheader()
    writer.writerow({'first_name': 'Baked', 'last_name': 'Beans'})
    writer.writerow({'first_name': 'Lovely', 'last_name': 'Spam'})
    writer.writerow({'first_name': 'Wonderful', 'last_name': 'Spam'})

answered Sep 4 at 7:39

G M

22.8k11 gold badges89 silver badges94 bronze badges

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