The warning ResourceWarning: unclosed file occurs when a file is opened but not properly closed in Python. If a file is left unclosed, it may lead to memory leaks, locked files, or unexpected behavior in certain operating systems.
1. Why Does This Happen?
- Forgetting to Close the File → Using
open()withoutclose(). - Exceptions Prevent Closing → If an error occurs before
file.close()is executed. - Garbage Collection Delay → In some Python versions, files are not closed immediately after use.
2. Common Causes and Solutions
Cause 1: Forgetting to Close the File
If you open a file but don’t close it manually, Python warns about an unclosed file.
Example (File Not Closed)
file = open("example.txt", "r")
data = file.read()
# Forgot to close the file!
Solution: Close the File After Use
file = open("example.txt", "r")
data = file.read()
file.close() # Manually closing the file
Cause 2: Not Using with open()
If an exception occurs before file.close(), the file remains unclosed.
Example (Error Before Closing)
file = open("example.txt", "r")
data = file.read()
raise Exception("Something went wrong!") # File won't be closed
file.close() # This won't execute
Solution: Use with open() (Auto-Closing Mechanism)
Using with open() ensures that Python automatically closes the file after exiting the block.
with open("example.txt", "r") as file:
data = file.read() # File closes automatically after the block
- This is the best practice to prevent
ResourceWarning.
Cause 3: Unclosed File in a Function
If a function opens a file and doesn’t close it, a ResourceWarning may appear.
Example (Function Doesn’t Close File)
def read_file():
file = open("example.txt", "r")
return file.read() # File remains open
print(read_file()) # Warning: unclosed file!
Solution: Use with open() in Functions
def read_file():
with open("example.txt", "r") as file:
return file.read() # File automatically closes
print(read_file()) # No warning
Cause 4: Delayed Garbage Collection
In some Python versions (e.g., Python 3.6+), files are not immediately closed when they go out of scope.
Example (Relying on Garbage Collection)
def open_file():
file = open("example.txt", "r")
return file.read() # File stays open in some cases
data = open_file()
Solution: Explicitly Close the File
def open_file():
with open("example.txt", "r") as file:
return file.read() # File is closed automatically
data = open_file()
Alternative: Manually Call Garbage Collection
import gc
gc.collect() # Forces Python to release unclosed files
- This is useful only if third-party libraries leave files open.
Cause 5: Using Open Files in Iterators or Generators
If a generator function yields from an open file, it may not close the file.
Example (Generator Keeping File Open)
def read_lines():
file = open("example.txt", "r")
for line in file:
yield line # File stays open while iterating
gen = read_lines() # File is still open
Solution: Use with open() Inside the Generator
def read_lines():
with open("example.txt", "r") as file:
for line in file:
yield line # File closes automatically
gen = read_lines() # No warning
3. Summary of Fixes
| Issue | Fix |
|---|---|
| Forgetting to close a file | Use file.close() or with open() |
Errors before file.close() | Use with open() to auto-close |
| Functions leaving files open | Use with open() inside functions |
| Delayed garbage collection | Manually call gc.collect() |
| Iterators keeping files open | Use with open() inside generators |