Python Error handling

Errors keep on going up until it gets caught

try:
    try:
        raise IndexError()
    except IntegrityError:
        print("Doesn't run")
except IndexError:
    print("Runs because it goes up until it gets caught")

Context Manager

Create your own context manager

with open('hello.txt', 'w') as f:
    f.write('hello')

same as

f = open('hello.txt', 'w')
try:
    f.write('hello')
finally:
    f.close()

Create your own class

• implement __enter__ and __exit__

class ManagedFile:
    def __init__(self, name):
        self.name = name
    def __enter__(self):
        self.file = open(self.name, 'w')
        return self.file  # the return value; ManagedFile() as f
    def __exit__(self, exception_type, exception_value, exception_traceback):
        if self.file:
            self.file.close()

with ManagedFile('hello.txt') as f:
    f.write('hello')

Using the contextmanager generator

from contextlib import contextmanager

@contextmanager
def managed_file(name):
    try:
        f = open(name, 'w')
        yield f  # gives f back to the caller, then can do f.write('hello')
    finally:
        f.close()

Python Tricks Page 33 context manager exercise

with Indenter() as indent:
    indent.print('hi!')
    with indent:
        indent.print('hello')
        with indent:
            indent.print('bonjour')
    indent.print('hey')

Should give

hi!
    hello
        bonjour
hey

class Indenter:
    def __init__(self):
        self.indents = -1
    def __enter__(self):
        self.indents += 1
        return self
    def __exit__(self, exception_type, exception_value, exception_traceback):
        self.indents -= 1
    def print(self, string):
        tab = "\t"
        print(f'{tab * self.indents}{string}')

Python Tricks Page 35 context manager

• context manager that measures the execution time of a code block using the time.time function

Context managers vs decorators

• separate concepts
• contextlib.contextmanager lets your context managers be decorators
  • so you don't need to use with

suppress

Cleaner version of try/except and ignore

from contextlib import suppress

with suppress(FileNotFoundError):
    os.remove('somefile.tmp')

same as

try:
    os.remove('somefile.tmp')
except FileNotFoundError:
    pass

Get the error message from an exception

• no standard property
• use str(e)
• or debug and get the properties for that specific Exception
  • error_message = e.detail["name"][0]

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Last update: 2023-04-24