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cells:
- markdown: |
# Advanced Python: `with` statements
### Prabhu Ramachandran
### The FOSSEE Python group &
### Department of Aerospace Engineering
### IIT Bombay
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- markdown: |
## Introduction
- `with` statement is used as follows
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- code: |
with open('data/test.txt') as f:
for line in f:
print(line, end='')
- code: |
f.closed
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- markdown: |
## Observations
- Creates a context
- `f` is automatically closed
- Works even if there was an exception!
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- markdown: |
## Use cases
- Use it with locks (for threaded computing)
- Database connections
- File objects
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- markdown: |
## Creating your own
- Relatively easy to do
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- code: |
from contextlib import contextmanager
@contextmanager
def context():
print("setup")
try:
yield
finally:
print("done")
- code: |
with context():
print("do something")
- markdown: |
## A more complex example
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slide_type: slide
- code: |
import os
from contextlib import contextmanager
@contextmanager
def tempfile(filename):
f = open(filename, 'w')
try:
yield f
finally:
f.close()
os.remove(filename)
- code: |
%ls junk.txt
- code: |
with tempfile('junk.txt') as f:
f.write('hello world\n')
f.flush()
metadata:
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slide_type: slide
- markdown: |
## Summary
- Useful feature
- Can define an object supporting this
- Overload, `__enter__` and `__exit__`
- Not covered here
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- markdown: |
## More information
- https://www.python.org/dev/peps/pep-0343
- [With statement](https://docs.python.org/3/reference/datamodel.html#with-statement-context-managers)
- [Context manager types](https://docs.python.org/3/library/stdtypes.html#typecontextmanager)
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