CODEWITHSUNDEEP
pythonfileionull
Bramat
Bramat

Reputation: 997

Writing "None" to a file in python

I have a txt file that contains rows of JSON objects. I'm parsing this file in Python, and I'm writing a file that every row in it will be a record (Comma separated) I've built from the JSON object.

Now, when I'm building this file - some of the values can be Null (or None in Python). So this is what I write:

a = 'abc'
b = None
str = a + "," + b
file.write(str+\n)

But I keep getting this error: TypeError: coercing to Unicode: need string or buffer, NoneType found

So my question is - How can I write "Null" values into the file, in a string, so when I load the file into the table - the value in that position will actually be Null? How can I keep this value in a file?

Thank you!

Upvotes: 4

Views: 9273

Answers (2)

Laurent LAPORTE
Laurent LAPORTE

Reputation: 22962

If it's Ok for you to have an empty string when you have None value, you can write:

a = 'abc'
b = None
row = a + "," + (b or "")
file.write(row + "\n")

Please, don't use str as a variable name because you shadow a built-in function/class

Or more generally, if you have a list of items:

items = ['abc', None]
row = [(item or "") for item in items]
file.write(",".join(row) + "\n")

Or use the CSV module.

With JSON, you could also have integers, list and dict. To convert that in string for a serialization in CSV, you could use:

def to_string(obj):
    if obj is None:
        return ""
    elif isinstance(obj, (list, dict)):
        raise TypeError(repr(type(obj)))
    else:
        return str(obj)

row = [to_string(item) for item in items]
file.write(",".join(row) + "\n")

Here, list and dict serialization is prohibited.

Upvotes: 2

cs95
cs95

Reputation: 402603

Use str.format. 

a = 'abc'
b = None
file.write("{},{}\n".format(a, b))

The error you get is in the line above file.write. When concatenating items to form a string, all items being concatenated must also be a string. This would mean, you'd need to do something along the lines of:

string = a + ',' + str(b)
file.write(string + '\n')

It is worth noting that you shouldn't use str, dict, or list to name variables as it shadows the inbuilt classes with the same name.

You might also consider using NaN, or, as mentioned just have an empty string "" instead of None. Furthermore, look into the csv module on how to write to a CSV file. Here's an example from the csv documentation:

import csv
with open('eggs.csv', 'w', newline='') as csvfile:
    spamwriter = csv.writer(csvfile, delimiter=',')
    spamwriter.writerow(['Spam'] * 5 + ['Baked Beans'])
    spamwriter.writerow(['Spam', 'Lovely Spam', 'Wonderful Spam'])

Upvotes: 2

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