Lecture 8

Context Managers

MCS 275 Spring 2023
David Dumas

Lecture 8: Context Managers

Reminders and announcements:

• Project 1 due Fri Feb 10 at 6pm central.
• Project 1 autograder will open soon.
• Homework 4 will be posted Thursday night.

Puzzle

What's the output?

        A = 2
        B = [3,4,5]
        C = [5,6,7]
        
        def f():
            A = 7
            B[0] = 7
            C = [7,7,7]
        
        f()
        print(A)
        print(B[0])
        print(C[0])
    

Puzzle

What's the output?

        A = 2
        B = [3,4,5]
        C = [5,6,7]
        
        def f():
            A = 7       # new local A
            B[0] = 7    # B.__setitem__(0,7) on global B
            C = [7,7,7] # new local C
        
        f()
        print(A)    # 2  (unchanged)
        print(B[0]) # 7  (same B, new item at index 0)
        print(C[0]) # 5  (unchanged)
    

Markdown

Text cells (Colab) or markdown cells (Jupyter) contain formatted text. When editing, formatting is specified with a language called Markdown.

# Heading level 1
## Heading level 2
### Heading level 3

* Bullet list item
* Another bullet list item

1. Numbered list item
1. Another numbered list item

Links: [text to display](https://example.com)

Common pattern

Acquire, use, and free a resource.

        x = resource() # open file, connect to database, ...

        x.action()
        x.action2(y,z)
        if x.status() == w:
            ...

        x.close() # or "release" or "delete"
    

Example: File I/O

# ACQUIRE
fp = open("data.txt","w",encoding="UTF-8")
# USE
for s in L:
    fp.write(s+"\n")
# RELEASE
fp.close()

Possible bug

Is the resource always freed? What if an exception is raised?

All files are closed when a program exits, but open files are a limited resource.

Will this function always close the file?

def file_contains_walrus(fn):
    """Return True if "walrus" is a line of file `fn`"""
    fileobj = open(fn,"r",encoding="UTF-8")
    for line in fileobj:
        if line.strip() == "walrus":
            fileobj.close()
            return True
    return False        

Currently, in CPython (the usual interpreter): Yes.

In CPython, local variables are deleted as soon as a function returns. Deleting a file object closes the file.

But this isn't a language guarantee!

Another way

Use with block to ensure automatic file closing.

with open("data.txt","w",encoding="UTF-8") as fileobj:
    fileobj.write(...)
    fileobj.write(...)
    # other write operations...
print("At this point, the file is already closed")

Extra bonus: you can see exactly what part of the program needs the open file.

Cleanup guarantee

A file opened using a with block will be closed as soon as execution leaves the block, even if an exception is raised.

Recommendation

Always open files using with, and make the body as short as possible.

Think of files like refrigerators: Open them for the shortest time possible.

In other languages

Other OO languages often recommend RAII:
Resource Acquisition is Instantiation.

Making an instance of a class acquires a resource, which is held for the lifetime of the object.

The resource is then freed by the class destructor when the object is deleted.

Python object lifetime

Python deletes objects you can no longer access (garbage collection) but:

• No promises about exactly when
• No guarantee any function (destructor) gets called as part of deletion
• Manual deletion is discouraged

Thus Python's with blocks are a substitute for RAII.

Context managers

Any object whose class is a context manager can be used in a with-block.

A context manager is a class with special methods:

• __enter__ to perform setup
• __exit__ to perform cleanup

Examples

Context managers are appropriate for:

• Network connections
• Database connections
• Locks
• Any limited or exclusive access right
• Temporary setup or changes that must be reverted

Context manager protocol

• __enter__(self):
  • Performs setup
  • Return value is assigned to the name after as in with statement.
• __exit__(self,exc_type,exc,tb):
  • Performs cleanup.
  • The arguments describe any exception that happened in the with block.

Expect each method to be called exactly once.

Built-in context managers

Some examples (listed as class - resource)

• open - Open file
• threading.Lock - Thread-exclusive right
• urllib.request.urlopen - HTTP connection
• tempfile.TemporaryFile - Temporary file (deleted after use)