Python

Print traceback call stack in Python

2018-04-172019-10-21VintaPython

Extract Traceback From An Exception

try:
    do_shit()
except Exception as exc:
    print('----- start -----')
    tb = _hx_e.__traceback__
    raise RuntimeError().with_traceback(tb)
    print('----- end -----')

reef:
https://stackoverflow.com/questions/11414894/extract-traceback-info-from-an-exception-object

Print Traceback Without Raising An Exception

print('----- start -----')
import traceback; traceback.print_stack()
print('----- end -----')

# or

import traceback
for line in traceback.format_stack():
    print(line.strip())

The result would be like:

>>> import qingcloud.iaas
>>> conn = qingcloud.iaas.connect_to_zone('pek2', '123', '456')
>>> conn.describe_instances(limit=1)
----- start -----
  File "<stdin>", line 1, in <module>
  File "qingcloud/iaas/connection.py", line 214, in describe_instances
    return self.send_request(action, body)
  File "qingcloud/iaas/connection.py", line 42, in send_request
    resp = self.send(url, request, verb)
  File "qingcloud/conn/connection.py", line 245, in send
    request.authorize(self)
  File "qingcloud/conn/connection.py", line 156, in authorize
    connection._auth_handler.add_auth(self, **kwargs)
  File "qingcloud/conn/auth.py", line 118, in add_auth
    import traceback; traceback.print_stack()
----- end -----

ref:
https://stackoverflow.com/questions/3925248/print-python-stack-trace-without-exception-being-raised

Find circular imports in Python

2018-03-282019-10-22VintaPython

What is circular imports?
http://stackabuse.com/python-circular-imports/

You could use python -vv to inspect import relations.

$ python -vv manage.py shell
>>> from api.models import Application
>>> from member.views.site import signup

$ python -vv
>>> import os
>>> os.environ.setdefault('DJANGO_SETTINGS_MODULE', 'streetvoice.settings')
>>> import django
>>> django.setup()
>>> from api.models import Application

ref:
https://stackoverflow.com/questions/6351805/cyclic-module-dependencies-and-relative-imports-in-python
https://stackoverflow.com/questions/9098787/tool-for-pinpointing-circular-imports-in-python-django

Python 3.7 has new feature to show time for importing modules. This feature is enabled with -X importtime option or PYTHONPROFILEIMPORTTIME=1 environment variable.

$ python3.7 -X importtime -c "import pipenv"

ref:
https://dev.to/methane/how-to-speed-up-python-application-startup-time-nkf

Dot notation (obj.x.y.z) for nested objects and dictionaries in Python

2018-02-272019-10-22VintaPython

Simple implementations of nested_getattr(obj, attr, default), nested_setattr(obj, attr, val) and nested_dict_get(dictionary, dotted_key).

Nested Object

import functools

raise_attribute_error = object()
def nested_getattr(obj, attr, default=raise_attribute_error):
    try:
        return functools.reduce(getattr, attr.split('.'), obj)
    except AttributeError:
        if default != raise_attribute_error:
            return default
        raise

# only support setting an attribute on an existed nested attribute
def nested_setattr(obj, attr, val):
    pre, _, post = attr.rpartition('.')
    return setattr(nested_getattr(obj, pre) if pre else obj, post, val)

nested_getattr(user, 'settings.enable_nsfw', None)

nested_setattr(user, 'settings.enable_nsfw', True)  # work
nested_setattr(user, 'nonexistent_field.whatever_field', True)  # raise AttributeError

ref:
https://stackoverflow.com/questions/11975781/why-does-getattr-not-support-consecutive-attribute-retrievals
https://stackoverflow.com/questions/31174295/getattr-and-setattr-on-nested-objects

Nested Dictionary

import functools

def nested_dict_get(dictionary, dotted_key):
    keys = dotted_key.split('.')
    return functools.reduce(lambda d, key: d.get(key) if d else None, keys, dictionary)

user_dict = {
    'id': 123,
    'username': 'vinta',
    'settings': {
        'enable_nsfw': True,
    },
}

nested_dict_get(user_dict, 'username')  # return 'vinta'
nested_dict_get(user_dict, 'settings.enable_nsfw')  # return True
nested_dict_get(user_dict, 'settings.notification.new_follower')  # return None

ref:
https://stackoverflow.com/questions/25833613/python-safe-method-to-get-value-of-nested-dictionary