Question

Here’s what I have:

import csv

a=8   
print a    
mylist = [a,'2','3']    
myfile = open("myfile.csv", "wb") # csv files should always be opened in binary mode 
wr = csv.writer(myfile, quoting=csv.QUOTE_ALL)    
wr.writerow(mylist)    
a = a + 1    
print a    
wr.writerow(mylist)

The result of the prints are 8 and 9, just as I would expect. When I open the file I created I have the result:

"8","2","3"     
"8","2","3"

The top row is what I would expect, but the second row starts with "8" rather than "9". I understand I can work around this by inserting mylist = [a,'2','3'] again after I redefine the variable, but would someone mind explaining to me why I have to reinsert the list line or why the variable isn’t automatically updated in the list? Is there another approach I can use to avoid having to reinsert the list line each time I want to update a variable?

Answer 1

It is the line:

a = a + 1

in which you reassigned a to a new integer, which is not connected to its previous value.

Anyway, integers are immutable variables and you cannot do anything else with it than reassign.

As @thg435 comments, list operation some_list = some_list + [5] would have the same effect. However, lists are mutable objects and you can use some_list.append(5), which would modify the list and reflect its changes into the csv file.

If you work with mutable objects (list, dict), you can modify their values:

import csv
a=8   
mylist = [a,'2','3']    
with open("myfile.csv", "wb") as myfile:
    wr = csv.writer(myfile, quoting=csv.QUOTE_ALL)    
    wr.writerow(mylist)    
    mylist[0] = mylist[0] + 1  ####  here you modify a list
    wr.writerow(mylist)