Working with Python context managers (with statement)
Jul 15, 2025 am 01:41 AM
The context manager is a mechanism in Python that simplifies resource management and ensures the correct release of resources, and is implemented through with statements. The core is to define the operations before and after the code is executed, and to automatically execute the two stages of "enter" and "exit", such as automatically closing the file or releasing the lock. The implementation methods include: 1. Use the class to define the __enter__() and __exit__() methods; 2. Use the @contextmanager decorator of the contextlib module to separate the entry and exit logic through yield. It has a wide range of application scenarios, such as file operation, database connection, locking mechanism, temporary directory management and performance timing, which can effectively improve code readability and security.
Using Python's with statements to process context managers is actually to make resource management safer and code clearer. The key behind it lies in the automatic control of the two stages of "entry" and "exit", such as automatically closing after opening a file, or automatically releasing after locking.
What is a context manager?
Simply put, a context manager is an object that defines what to do before and after a piece of code is executed. The most common example is to open a file with open(...) as f: There is no need to call f.close() manually, Python will automatically close it for you when with block ends.
Implementationally, a context manager needs to support two methods:
-
__enter__(): Execute when enteringwithblock, the return value is usually assigned to the variable afteras. -
__exit__(): is automatically called at the end ofwithblock and will be executed regardless of whether an exception occurs.
These two methods allow you to do some cleaning work, such as closing connections, freeing resources, etc.
How to customize a context manager?
You can create your own context manager via classes or decorators.
Method 1: Implement with classes
class MyContext:
def __enter__(self):
print("Get into context")
Return self
def __exit__(self, exc_type, exc_val, exc_tb):
print("Exit context")
# Return True to suppress exceptions. Return True
with MyContext() as mc:
print("Execution")This method is suitable for situations where the structure is relatively fixed and multiple methods are required to cooperate.
Method 2: Use contextlib decorator
If you don't want to write classes, you can use the @contextmanager decorator provided by the standard library contextlib :
from contextlib import contextmanager
@contextmanager
def my_context():
print("enter")
try:
yield
Finally:
print("Exit")
with my_context():
print("Execute") The core here is yield , the upper part is equivalent to __enter__() , and the lower part is equivalent to __exit__() .
What can a context manager be used for?
In addition to file operations, context managers are useful in many places:
- Database connection management : Connect to the database when entering, and automatically disconnect or commit transactions when exiting.
- Lock mechanism : acquire the lock when entering and release the lock when exiting to avoid deadlock.
- Temporary directory/file management : Create a temporary directory when entering and automatically clean up when exiting.
- Performance timing : Record the entry time, and output time is taken when exiting.
To give a simple example, you want to count the time a piece of code runs:
import time
from contextlib import contextmanager
@contextmanager
def timer():
start = time.time()
yield
end = time.time()
print(f"Time-consuming: {end - start:.2f} seconds")
with timer():
time.sleep(1)This way you don't need to manually write the start and end times each time.
Basically that's it. Making good use of context managers can make resource management and exception handling concise and reliable. The key is to understand what to do in the entry and exit stages, and how to encapsulate logic in combination with actual scenarios.
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