Writing data from a Python List and a Dictionary to CSV

Question

Assume mydict is unique based on each list item:

 mylist = ['li1', 'li2']
 mydict =  {'key1': 'value1','key2': 'value2','key3': 'value3}

I want to write this stracture in a CSV file:

ListItem, key1, key2, key3
li1, value1, value2, value3
li2, value1, value2, value3

This is a sample of how I try to do this; but my code overwrites the first line with each iteration, and I do not know how to write the list item in the first column. Could you give me a hand, please?

import pandas as pd
import random


def CreateDict(li):
    dict = {}
    dict['x'] = random.randrange(1, li)  #25
    dict['y'] = random.randrange(1, li)  #27
    print(dict)
    return dict

mylist = [10, 20, 30]
for li in mylist:
    mydict = CreateDict(li)
    df = pd.DataFrame([mydict])
    df.to_csv('test.csv', index=False)

I get this as an output:

x,y
25,27

In the for loop the data frame and the csv file get overwritten with each iteration. Therefore the data frame as well as the csv file will only contain the dict created in the last iteration of the loop. — ai2ys
– ai2ys, Commented Jan 30, 2021 at 11:19

Shubham Sharma · Accepted Answer · 2021-01-30 11:50:06Z

4

Let's try dict.fromkeys to generate data dictionary, then create a dataframe from this data dictionary and use DataFrame.to_csv to save as a csv file:

data = dict.fromkeys(mylist, mydict)
pd.DataFrame(data).T.rename_axis('ListItem').to_csv('foo.csv')

EDIT: If you would like to create unique dictionary per item in mylist you can use:

data = {li:CreateDict(li) for li in mylist}
pd.DataFrame(data).T.rename_axis('ListItem').to_csv('foo.csv')

# foo.csv

ListItem,key1,key2,key3
li1,value1,value2,value3
li2,value1,value2,value3

edited Jan 30, 2021 at 11:50

answered Jan 30, 2021 at 10:53

Shubham Sharma

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7 Comments

Flodude Over a year ago

Thanks, I use the code in the loop, but the dictionary values are not changing in the CSV file.

Shubham Sharma Over a year ago

@Flodude You don't have to use the code in the loop

Shubham Sharma Over a year ago

@Flodude Not sure but i guess you want pd.DataFrame({li:CreateDict(li) for li in mylist}).T.rename_axis('ListItem').to_csv('foo.csv')..One more thing don't use dict as an identifier its reserved keyword in python.

Flodude Over a year ago

It works but it re-run the CreateDict function!

Shubham Sharma Over a year ago

@Flodude Can you please elaborate re-run the CreateDict function?

|

IoaTzimas · Accepted Answer · 2021-01-30 10:59:33Z

2

Working on your existing solution:

import pandas as pd
import random


def CreateDict(li):
    dict = {}
    dict['x'] = random.randrange(1, li)  #25
    dict['y'] = random.randrange(1, li)  #27
    print(dict)
    return dict

mylist = [10, 20, 30]
df=pd.DataFrame(columns=['x', 'y'])
for li in mylist:
    mydict = CreateDict(li)
    df=pd.concat([df, pd.DataFrame([mydict])])

df['mylist']=mylist
result=df[['mylist', 'x', 'y']]
result.to_csv('test.csv', index=False)

Output:

mylist  x  y
   10   6  3
   20   8  2
   30   7  4

answered Jan 30, 2021 at 10:59

IoaTzimas

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Comments

Salvatore Calcagno · Accepted Answer · 2021-01-30 11:00:24Z

0

First of all you should create a dictionary outside the function, add items in loop and write only at the end. With your code you're writing 3 times the same file and you see only the last row.

For index you should set index=True.

Here the code. Hope I help you!

import pandas as pd
import random

my_dict = {
    'key1':[],
    'key2':[],
    'key3':[]
}

def AddToDict(li):
    my_dict['key1'].append(random.randrange(1, li))
    my_dict['key2'].append(random.randrange(1, li))
    my_dict['key3'].append(random.randrange(1, li))

mylist = [10, 20, 30]
for li in mylist:
    AddToDict(li)

df = pd.DataFrame(my_dict)  # Here
df.to_csv('test.csv', index=True)

answered Jan 30, 2021 at 11:00

Salvatore Calcagno

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1 Comment

Flodude Over a year ago

Thank you, but it doesn't write the list item in the first row.

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