不使用换行符Python将列表写入文件

Question

I have the following code, and I simply want it to write the list to a file. It does so, but with a blank line each time, which causes problems with every iteration!

with open("films.txt", mode='r') as f:
 allfilms=[]

 reader = csv.reader(f)
 for row in reader:
    #create a list again which reads in and stores all films in the file

    allfilms.append(row)
    print(allfilms)


with open("films.txt","w") as f:
     writer=csv.writer(f)
     writer.writerows(allfilms)

The output is: 0,Genre, Title, Rating, Likes

    1,Sci-Fi,Out of the Silent Planet, PG, 0

    2,Sci-Fi,Solaris, PG,0

    3,Sci-Fi,Star Trek, PG, 0

    4,Sci-Fi,Cosmos, PG, 0

As mentioned, I don't want the blank line when saving the list. Also, I'd be interested in more elegant or better ways of doing the same thing. I want the file to be READ in to the list, and then written back to the file EXACTLY the same, in terms of formatting, as what was stored in the file.

Thanks in advance

Answer 1

Edit: I misunderstood how the csv module gave output when I wrote this answer, but I am not removing it due to the helpful comment by downshift on it. Please see my more recent and more accurate answer.

downshift answered this in the comments, but I will try to give a little more detail about why this is happening.

When you read each row in from the csv reader, it gives you the entire row, which includes the newline character ( \\n ) at the end. Your issue stems from the fact that when you write to the file, the write function adds it's own newline character to the end of your string. Luckily there are two very simple ways to remove the newline character from the end of the line you get back from the csv reader.

One way is to use the .strip() method of a string, which takes in some characters in the form of a string as input, and strips of as many of those characters as possible from the ends of the string. Using this method you could replace:

allfilms.append(row)

with

allfilms.append(row.strip("\n"))

Another way you could do it is to use string slicing and slice off the last character of the string. I wouldn't recommend you use string slicing for this, because it is less clear what you are doing, but if you wanted to, you would replace:

allfilms.append(row)

with

allfilms.append(row[:-1])

row[:-1] will return row with the last character excluded, discarding the newline character.

Hope this helped you!

Answer 2

I believe the source of your issue is that when you opened films.txt , you forgot to include the newline='' argument as the docs say to use. This means your code becomes:

with open("films.txt", mode='r', newline='') as f:

and

with open("films.txt", mode='w', newline='') as f: