Writing a python Dictionary to a CSV file with keys as column headers

Question

I'm trying to write the elements in my dictionary into a text file where each key would be a column. Currently have I something that looks like

import csv
import numpy as np



data1 = np.arange(10)
data2 = np.arange(10)*2
data3 = np.arange(10)*3

writefile = '../Desktop/data.txt'
datadict = {} 

datadict['data1'] = data1
datadict['data2'] = data2
datadict['data3'] = data3


f = open( writefile, 'w' )
fieldnames = ['data1','data2', 'data3']
data = csv.DictWriter(writefile, fieldnames, restval='', extrasaction='ignore', dialect='excel')

f.close()

but it gives me the error "argument 1 must have a "write" method". I'm not sure what that means. I'm also worried about the dialect = 'excel', but I'm not sure what else to put. In the end I'd like a file that has something looking like:

enter image description here

Thanks

score 8 · Accepted Answer · answered Oct 01 '12 at 21:54

No need to use DictWriter here at all:

import csv
import numpy as np

data1 = np.arange(10)
data2 = np.arange(10)*2
data3 = np.arange(10)*3

writefile = '../test.csv'
fieldnames = ['data1','data2', 'data3']
with open( writefile, 'w' ) as f:
    writer = csv.writer(f)
    writer.writerow(fieldnames)
    writer.writerows(zip(data1, data2, data3))

defuz · Answer 2 · 2015-06-03T07:43:35.727

2

You should pass the file-like object as the first argument to the DictWriter constructor.

datalist = [{'data1': d1, 'data2': d2, 'data3': d3} for d1, d2, d3 in zip(data1, data2, data3)]
...
f = open(writefile, 'w')
...
writer = csv.DictWriter(f, ...) 
for i in xrange(10):
    writer.writerow(datalist[i])
f.close()

More simple analogic case:

with open(writefile, 'w') as f:
    writer = csv.writer(f)
    for row in zip(data1, data2, data3):
        writer.writerow(row)

edited Jun 03 '15 at 07:43

answered Oct 01 '12 at 20:57

defuz

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This gives me an error since the file doesn't already exist. When I put an empty file there it returns the empty file. – Alexa Halford Oct 01 '12 at 21:00
fixed, try again) (I forgot `'w'` modificatory, and you should close the file or flush it) – defuz Oct 01 '12 at 21:01
I did and still I end up with an empty file – Alexa Halford Oct 01 '12 at 21:02
we forgot to use `writer.writerow` (http://docs.python.org/library/csv.html#csv.writer). This should work. – defuz Oct 01 '12 at 21:16
I'll try that for loop after diner, but that looks promising. thanks. – Alexa Halford Oct 01 '12 at 22:36
The with statement is wrong, it should be `with open(writefile, 'w') as f:` – Michael Jun 03 '15 at 07:29

score 1 · Answer 3 · edited May 23 '17 at 12:13

If you're doing the kind of data analysis it looks like you might be doing here, the best bet might be to get into pandas, Python's dedicated data analysis library.

Here's an example which does what you want:

import pandas as pd
import numpy as np

data1 = np.arange(10)
data2 = np.arange(10)*2
data3 = np.arange(10)*3

df = pd.DataFrame(zip(data1, data2, data3), columns=['data1', 'data2', 'data3'])
df.to_csv('myfile.csv')

Pretty neat I'd say. Additionally, you can now do all sorts of unimaginable things with your data.

You can also make a DataFrame from an existing dictionary:

In [115]: a_dict = {'foo':[1,2,3], 'bar':[4,5,6], 'baz':[7,8,9]}

In [116]: pd.DataFrame(a_dict)
Out[116]: 
   bar  baz  foo
0    4    7    1
1    5    8    2
2    6    9    3

Abhinav Sarkar · Answer 4 · 2012-10-01T21:13:01.770

0

Try this:

import csv
import numpy as np

data1 = np.arange(10)
data2 = np.arange(10)*2
data3 = np.arange(10)*3

writefile = '../Desktop/data.txt'
datadictlist = [{'data1': data1[i], 'data2': data2[i], 'data3': data3[i]} for i in xrange(10)]

f = open( writefile, 'w' )
fieldnames = ['data1','data2', 'data3']
writer = csv.DictWriter(f, fieldnames, restval='', extrasaction='ignore', dialect='excel')
writer.writerows(datadictlist)

f.close()

edited Oct 01 '12 at 21:13

answered Oct 01 '12 at 20:55

Abhinav Sarkar

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That just gives me an empty file without any data written in it. – Alexa Halford Oct 01 '12 at 20:57
It's now printing data (yea), but not in the right format. It's giving me one line with [0 1 2 3 4 5 6 7 8 9],[ 0 2 4 6 8 10 12 14 16 18],[ 0 3 6 9 12 15 18 21 24 27] – Alexa Halford Oct 01 '12 at 21:07
@AlexaHalford, that is because you are assigning the whole array to a single field in one dictionary. I updated the code to print the output you want to show. – Abhinav Sarkar Oct 01 '12 at 21:14

score 0 · Answer 5 · edited Oct 02 '12 at 00:42

0

put 'a+' instead of 'w'

f = open( writefile, 'a+' )

edited Oct 02 '12 at 00:42

andrewsi

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answered Oct 01 '12 at 21:54

nickanor

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Writing a python Dictionary to a CSV file with keys as column headers

5 Answers5