ValueError: I/O operation on closed file — how to fix

Traceback (most recent call last):
  File "app.py", line 6, in <module>
    f.write("more\n")
ValueError: I/O operation on closed file.

Quick fix: Move every read/write inside the with open(...) as f: block (check the indentation), or reopen the file. For csv writers, keep writer.writerow() inside the with. For io.StringIO/BytesIO, call getvalue() before closing.

What it means

A file object becomes unusable once close() runs. Any later read(), write(), readline(), iteration or seek() raises ValueError: I/O operation on closed file. A with block closes the file automatically when the indented block ends, so the most common cause is a line that is indented one level too little.

You can check the state: f.closed is True after closing.

Cause 1: code outside the with block

with open("out.txt", "w") as f:
    f.write("first\n")
f.write("second\n")      # ValueError: file already closed

Fix: indent the code so it runs while the file is open.

with open("out.txt", "w") as f:
    f.write("first\n")
    f.write("second\n")

If you need the data later, read it into a variable inside the block:

with open("data.txt") as f:
    lines = f.readlines()
for line in lines:        # fine: works on the list, not the file
    print(line)

Cause 2: csv writer/reader used after the file closed

csv.writer and csv.reader wrap the file object; they don't keep it open.

import csv

with open("rows.csv", "w", newline="") as f:
    writer = csv.writer(f)
writer.writerow(["a", "b"])   # ValueError

Fix: do all writerow() calls inside the with:

with open("rows.csv", "w", newline="") as f:
    writer = csv.writer(f)
    for row in rows:
        writer.writerow(row)

Cause 3: returning a file or generator from a function

def read_lines(path):
    with open(path) as f:
        return (line.strip() for line in f)   # generator runs after close

for line in read_lines("log.txt"):   # ValueError
    ...

Fix: either materialize the data before returning, or make the function itself a generator so the file stays open while you iterate:

def read_lines(path):
    with open(path) as f:
        for line in f:
            yield line.strip()

Cause 4: closing explicitly too early

A stray f.close() inside a loop closes the file after the first iteration:

f = open("out.txt", "w")
for item in items:
    f.write(item + "\n")
    f.close()            # wrong place

Move close() after the loop, or better, use with.

Cause 5: StringIO / BytesIO and libraries that close for you

io.StringIO.getvalue() fails once the buffer is closed. Get the value first:

import io
buf = io.StringIO()
buf.write("hello")
data = buf.getvalue()
buf.close()

Some libraries close the stream they are given. For example pandas.to_csv(buf) doesn't close it, but wrapping a buffer in io.TextIOWrapper and letting the wrapper be garbage-collected closes the underlying buffer too. Call wrapper.detach() before discarding the wrapper if you still need the buffer.

In web frameworks (Flask, Django, FastAPI), an uploaded file is closed when the request ends. Read or save it during the request (file.save(path) or data = await file.read()), not in a background task that runs later.

Cause 6: sys.stdout was closed or replaced

If something called sys.stdout.close() (or a with block wrapped sys.stdout), every later print() raises this error. Don't use with sys.stdout:; use contextlib.redirect_stdout to redirect output temporarily.

Last updated 2026-11-07.