Save a list to a file

问题

I have a library where I want to create a new book and then add it to my list of books. What I have problems with is to save the file between calls.

This is how I read the file:

def read_bookfile():

    try:
        booklibrary_file = open("a.txt")

        booklibrary_list = []
        booklist = booklibrary_file.readlines()
        for rad in booklist:
            linelist = rad.split("/")
            title = linelist[0]
            firstname = linelist[1]
            lastname = linelist[2]
            isbn = int(linelist[3])
            availability = linelist[4]
            borrowed = linelist[5]
            late = linelist[6]
            returnday = linelist[7]
            b = Book(title, firstname, lastname, isbn, availability, borrowed, late, returnday) 
            booklibrary_list.append(b)
        booklibrary_file.close()
        return booklibrary_list

Now I want to know how to save to my file.

回答1:

In order to save to a file, you have to open it in Write-Append mode.

library_file = open("a.txt", "a")
...
library_file.write("Some string\n")
...
library_file.close()

Refer to Python's documentation on Built-in Functions for more information.

回答2:

First off, here's an easier way to read, assuming those eight fields are the only ones:

def read_bookfile(filename="a.txt"):
    with open(filename) as f:
        return [Book(*line.split('/')) for line in f]

Now, to save:

 def save_bookfile(booklist, filename='a.txt'):
     with open(filename, 'w') as f:
         for book in booklist:
             f.write('/'.join([book.title, book.firstname, book.lastname, str(book.isbn),
                               book.availability, book.borrowed, book.late, book.returnday])
                     + '\n')

assuming the Book model just saves those attributes in as they were passed (as strings).

Explanations:

• The with statement opens your file and makes sure that it gets closed when control passes out of the statement, even if there's an exception or something like that.
• Passing in the filename as an argument is preferable, because it allows you to use different filenames without changing the function; this uses a default argument so you can still call it in the same way.
• The [... for line in f] is a list comprehension, which is like doing lst = []; for line in f: lst.append(...) but faster to write and to run.
• Opening a file in 'w' mode allows you to write to it. Note that this will delete the already-existing contents of the file; you can use 'a' or 'w+' to avoid that, but that requires a little more work to reconcile the existing contents with your book list.
• The * in read_bookfile splits a list up as if you passed them as separate arguments to a function.
• '/'.join() takes the list of strings and joins them together using slashes: '/'.join(["a", "b", "c"]) is "a/b/c". It needs strings, though, which is why I did str(isbn) (because book.isbn is an int).

回答3:

Python is "batteries included", remember?

Consider using the "csv" module:

use csv

csv.reader(...)
csv.writer(...)

I think these have lots of options (like you can set your delimiters to be other than commas; you can read in to a list of dictionaries, etc.)

See Python Docs for CSV reader/writer:

回答4:

I have to make a few assumptions about your Book class, but I think this might help put you on the right track:

bookList = read_bookfile()

outfile = open("booklist.txt", "w")

for book in bookList:
        bookStr = book.title + " " + book.firstname + " " + book.lastname + " " + book.isbn + " " + book.availability + " " + book.borrowed + " " + book.late + " " + book.returnday + "\n"
        outfile.write(bookStr)

outfile.close()

来源：https://stackoverflow.com/questions/10419417/save-a-list-to-a-file