How to resolve KeyError: <variable> in Python?

Question

Hi I'm trying to open simple csv file with the header from an external file:

got next file named: name.csv with next content:

Leo,Days,Ju
Tomas,Lee,Bruce
Max,Perez,Smith

If I code:

import csv
sep = ','
with open('name.csv') as csvfile:
     fieldnames = ['name', 'paterno', 'materno']
     reader = csv.DictReader(csvfile,fieldnames)
     for row in reader:
         list = (row['name'], \
                 row['materno'])
         print (sep.join(list))

The result is desired like:

Leo,Ju
Tomas,Bruce
Max,Smith

But if got an extra file with headers named hdr_name.txt with:

['name', 'paterno', 'materno']

With this new code:

import csv
sep = ','
fieldnames =  open('hdr_name.txt', 'r').read()
with open('name.csv') as csvfile:
    print(fieldnames)
    reader = csv.DictReader(csvfile,fieldnames)
    for row in reader:
        list = (row['name'], \
            row['materno'])
        print (sep.join(list))

Got as result:

Traceback (most recent call last):
  File "<stdin>", line 5, in <module>
KeyError: 'name'

But if I ask for 'name' in fieldnames, is there!

>>> 'name' in fieldnames
True
>>>

What I'm doing wrong, with opening header from external file ?

Just read your header so fieldnames = open('hdr_name.txt', 'r').readlines() — amarynets
– amarynets, Commented Oct 4, 2017 at 21:54
@AndMar That unfortunately does not fix it, because the file contains a single line with a str'd list. — cs95
– cs95, Commented Oct 4, 2017 at 21:55
More importantly, stop dumping the string representation of an object to a txt file and calling it serialization - use pre-existing serialization formats. — juanpa.arrivillaga
– juanpa.arrivillaga, Commented Oct 4, 2017 at 21:59

cs95 · Accepted Answer · 2017-10-04 21:53:00Z

2

fieldnames is a string that looks like this:

"['name', 'paterno', 'materno']"

Naturally, a membership test will return true, but that does not imply fieldnames is a list. Remember, file.read returns a string - you still need to cast it to a list.

This doesn't appear to look like JSON, so I'd recommend ast:

import ast
with open('hdr_name.txt', 'r') as f:
     fieldnames = ast.literal_eval(f.read().strip())

answered Oct 4, 2017 at 21:53

cs95

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4 Comments

Israel Diaz Over a year ago

Ok. found other of the multiple ways to do it, once I got the main idea! Thanks @COLDSPEED fieldnames = eval(open('hdr_name.txt', 'r').read()) Using eval()

cs95 Over a year ago

@IsraelDiaz try to stay away from eval, it is unsafe compared to ast.

Israel Diaz Over a year ago

@COLDSPEED - Oky. got it! 10xs!

Israel Diaz Over a year ago

Just more info about it: from documentation states: See ast.literal_eval() for a function that can safely evaluate strings with expressions containing only literals. [ docs.python.org/2/library/functions.html?highlight=eval#eval ]

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