why do I get ZeroDivisionError?

Question

I am trying to write python program that reads a .csv file. I have in the input file 4 columns/fields and I want to only have 2 in the output with one being a new one.

This is the input I am using:

MID,REP,NAME,NEPTUN
0,,"people's front of judea",GM6MRT
17,,Steve Jobs,NC3J0K
,0,Brian,RQQCFE
19,9,Pontius Pilate,BQ6IAJ
1,,N. Jesus,QDMXVF
18,,Bill Gates,D1CXLO
0,,"knights who say NI",CZN5JA
 ,1,"Robin, the brave",BWQ5AU
17,19,"Gelehed, the pure",BY9B8G

then the output should be something like this(not full output):

NEPTUN,GROWTH
BQ6IAJ,-0.5263157894736842
BWQ5AU,inf
BY9B8G,0.11764705882352941
RQQCFE,0

The new field called GROWTH is calculated by (REP-MID)/MID.

So, I am using two lists to do that:

import csv

L = []
s =[]
with open('input.csv', 'rb') as R:
    reader = csv.DictReader(R)
    for x in reader:
        if  x['MID'] != '' or '0' and x['REP'] == '':
            Growth = -float(x['MID'])/float(x['MID'])
            L.append(x)
            s.append(Growth)
        elif  x['MID'] != '' or '0':
            Growth = (float(x['REP']))-float(x['MID'])/float(x['MID'])
            L.append(x)
            s.append(Growth)
        elif x['MID'] and x['REP'] == '' or '0' :
            Growth = 0
            L.append(x)
            s.append(Growth) 
        else:
            Growth = float("inf")
            L.append(x)
            s.append(Growth) 
    for i in range(len(s)):
        L[i]['GROWTH'] = i
R.close()
with open('output.csv', 'wb') as output:
    fields = ['NEPTUN', 'GROWTH']
    writer = csv.DictWriter(output, fieldnames=fields, extrasaction='ignore')
    writer.writeheader()
    writer.writerows(L)
output.close()

Now, I am not even sure if the code is correct or does what I aim it for, because I am stuck at a ZeroDivisionError: float division by zero at the first ifcondition and I tried many ways to avoid it but I get the same error.

I thought the problem is that when there are no values for MID field, I think the dictionary gives it `` value and that can't be transformed to 0 by float(). But it seems that is not the problem, but honestly I have no idea now, so that is why I am asking here.

the full error:

Growth = -float(x['MID'])/float(x['MID'])

ZeroDivisionError: float division by zero

Any hints about this are greatly valued.

What is `if x['MID'] != '' or '' and x['REP'] == '':` supposed to mean? — Barmar, May 25 '18 at 14:53
@Kadir.J `or '0'` doesn't actually mean anything, it just returns true as a string is always true, so it will always fall into your if statement — WhatsThePoint, May 25 '18 at 14:55
@barmar, `x['REP'] == ''` is for when REP is equal to nothing, because otherwise it gives me can not transform a float from a string error — JKM, May 25 '18 at 14:55
Should be `if x['MID'] != '' and x['MID'] != '0' and x['REP'] == '':` — Barmar, May 25 '18 at 14:55
Also related: https://stackoverflow.com/questions/26337003/execute-block-if-a-variable-is-not-one-of-some-specific-values — Barmar, May 25 '18 at 14:56

score 1 · Accepted Answer · answered May 25 '18 at 14:58

if x['MID'] != '' or '0' and x['REP'] == ''

This does not mean what you think it means. It is interpreted as

if ((x['Mid'] != '') or ('0')) and (x['REP'] == '')

which shortens to

if True and x['REP'] == ''

which in turn becomes

if x['REP'] == ''

What you mean is

if x['Mid'] not in ('', '0') and (x['REP'] == ''):

You need to do the same for your other if statements

why do I get ZeroDivisionError?

1 Answers1