Python3 write to file using Print

New version

with open(f'output.csv', 'w') as f:

      print(data, file=f)

Old version

with open(f'output.csv', 'w') as f:

      f.write(data)

python contextlib vs context manager

import contextlib

class contextManagerExample:
  def __init__(self):
    print('inside __init__')
  def __enter__(self):
    print('inside __enter__')
    return 'returning from contextManagerExample enter'
  def __exit__(self, exc_type, exc_val, exc_tb):
    print('inside __exit__')

#No need to write __enter__, __exit__ separately
#yield instead of return
@contextlib.contextmanager
def context_lib_test():
  try:
    yield 'returning from context_lib_test enter'
  except Exception as e:
    raise

if __name__ == '__main__':
  print('-'*20)
  with contextManagerExample() as cm:
    print('inside ContextManagerExample scope')
    print(cm)
  print('-'*20)
  with context_lib_test() as cm:
    print('inside context_lib_test scope')
    print(cm)
  print('-'*20)



Output:
--------------------
inside __init__
inside __enter__
inside ContextManagerExample scope
returning from contextManagerExample enter
inside __exit__
--------------------
inside context_lib_test scope
returning from context_lib_test enter
--------------------

python context managers using contextlib

"""
# Using contextlib you don't have to explicitly write __enter__, __exit__
# yield instead of return
"""
import contextlib
import sys
import time

@contextlib.contextmanager
def context_manager_def_test():
    print('context_manager_def_test: ENTER')
    try:
        yield 'You are in with-block'
        print('context_manager_def_test: NORMAL EXIT')
    except Exception:
        print('context_manager_def_test: EXCEPTION EXIT', sys.exc_info())
        raise

print('*'*75)

with context_manager_def_test() as cm:
    print('Inside ContextManagerTest')
    print(cm)

print('*'*75)
time.sleep(1)

with context_manager_def_test() as cm:
    print('Inside ContextManagerTest')
    print(cm)
    raise ValueError('something is wrong')

print('*'*75)


"""
***************************************************************************
context_manager_def_test: ENTER
Inside ContextManagerTest
You are in with-block
context_manager_def_test: NORMAL EXIT
***************************************************************************
context_manager_def_test: ENTER
Inside ContextManagerTest
You are in with-block
context_manager_def_test: EXCEPTION EXIT (<class 'ValueError'>, ValueError('something is wrong'), <traceback object at 0x1023ed0c8>)
Traceback (most recent call last):
  File "/Users/prabhathkota/Workspace/prabhath/personal/Python_Scripts/context_managers/contextlib_example.py", line 31, in <module>
    raise ValueError('something is wrong')
ValueError: something is wrong
***************************************************************************
"""

Python context manager with exceptions

###################
# __enter__
# __enter__ is called before executing with-statement body
# __exit__
# __exit__ called after with-statement body
# File opening is context managers
###################


class ContextManagerTest:
    def __init__(self):
        print('Inside __init__')

    def __enter__(self):
        print('Inside __enter__')
        return 'returning ... Inside with block'
        # return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        if exc_type is None:
            print('Inside __exit__ without exception')
        else:
            print('Inside __exit__ with exception ({} - {} - {})' .format(exc_type, exc_val, exc_tb))


with ContextManagerTest() as cm:
    print('Inside ContextManagerTest')
    print(cm)
    raise ValueError('something is wrong')


"""
Traceback (most recent call last):
Inside __enter__
  File "...../Python_Scripts/context_managers/context_manager_with_exception.py", line 30, in <module>
Inside ContextManagerTest
    raise ValueError('something is wrong')
returning ... Inside with block
ValueError: something is wrong
Inside __exit__ with exception (<class 'ValueError'> - something is wrong - <traceback object at 0x1034ba608>)

"""

Python context manager

###################
# __enter__
# __enter__ is called before executing with-statement body
# __exit__
# __exit__ called after with-statement body
# File opening is context managers
###################


class ContextManagerTest:
    def __init__(self):
        print('Inside __init__')

    def __enter__(self):
        print('Inside __enter__')
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        if exc_type is None:
            print('Inside __exit__ without exception')
        else:
            print('Inside __exit__ with exception ({} - {} - {})'.format(exc_type, exc_val, exc_tb))
        return


with ContextManagerTest() as cm:
    print('Inside ContextManagerTest')
    print(cm)


"""
Inside __init__
Inside __enter__
Inside ContextManagerTest
<__main__.ContextManagerTest object at 0x10a920160>
Inside __exit__ without exception
"""