Python Notes

python.py

# debugging
import pdb
pdb.set_trace()

# override a class method in a subclass (Python 2.x)
class Foo(object):  
  @classmethod
  def baz(cls, x):
    return x + 3

class Bar(Foo):
  @classmethod
  def baz(cls, x):
    return super(Bar, cls).baz(x) + 3
  
# parse & pretty print JSON
import json
print(json.dumps(json.loads(data), indent=4))

# imports: http://bhfsteve.blogspot.com/2012/06/patching-tip-using-mocks-in-python-unit.html

# this approach actually imports baz into the namespace of the module that imported it, so
# its fully-qualified name is now: my.module.file.baz (assuming it was imported in: my/module/file.py)
from foo.bar import baz

# this approach imports the whole foo package (and all of its subpackages), so the fully-qualified
# name of the baz function is still foo.bar.baz
import foo

# monkey patching - you need to take the import style into account when monkey patching 

# in my.module.file.py
from foo.bar import baz
my.module.file.baz = monkey_baz

# in my.other.module.file.py
import foo
foo.bar.baz = monkey_baz

# dynamic method dispatch

class Foo:
  def bar(self, x):
    return x + 2
  
  def dispatch(self, method_name, *args):
    m = getattr(self, method_name)
    return m(*args)
  
# Generators calling generators

class Generator:
    def outer(self):
        for result in self.inner():
            print("outer yield: {}".format(result))
            for value in result["values"]:
                yield value

    def inner(self):
        for i in range(3):
            print("inner yield")
            yield {"values":range(i, 3)}
            
# in Python 3, yield from can be used to yield from can be used

class Generator:
    def outer(self):
        for i in range(3):
            print("outer yield from: {}".format(i))
            yield from self.inner(i)
    
    def inner(self, start_idx):
        for value in range(start_idx, 3):
            print("inner yield")
            yield value
        

for (i, val) in enumerate(Generator().outer()):
    print("{}: {}".format(i, val))
    
# gevent & exceptions

from gevent.pool import Pool

pool = Pool(16)

def flaky_func(arg):
    if (arg + 1) % 7 == 0:
        raise Exception("Problem")
    return (arg * 3) - 4

# this raises an exception as soon as imap iterates to a result that raised an exception
unsafe_results = pool.imap(flaky_func, range(100))
for (i, val) in enumerate(unsafe_results):
    print("{}: {}".format(i, val))

def safe_func(fn):
    def safe(*args, **kwargs):
        try:
            return fn(*args, **kwargs)
        except Exception as e:
            return None
    return safe

# this does not raise
safe_results = pool.imap(safe_func(flaky_func), range(100))
for (i, val) in enumerate(safe_results):
    print("{}: {}".format(i, val))
    
# can also use the built-in wrap_errors() function to catch the exception and return it as the value
from gevent.util import wrap_errors

safe_results = pool.imap(wrap_errors((Exception,), flaky_func), range(100))

# invoke a decorated method without going through its decorator 
# (ugly, but can be useful in cases like tests for a method with a retry decorator where you want to avoid retrying)

class Foo:
    @some_decorator
    def some_method(self):
        pass
  
result = Foo().some_method.__wrapped__()

# secondary constructor

class Foo(object):
    @classmethod
    def for_x(cls, x):
        val = cls.__new__(cls)
        val.bar = x
        return val
      
    def __init__(self):
        self.bar = "baz"
        
        
## Descriptors

# definition
class Descriptor:
  def __get__(self, obj, objtype=None):
    return "baz"
  
# usage
class Bar:
  foo = Descriptor() # must be stored as a class variable in another class
  
Bar.foo  # returns "baz"
  
  
## Print stack trace

import traceback

traceback.print_stack(file=sys.stdout)


## Reverse a string/array
"Some Text"[::-1]  # "texT emoS"

## Class methods, inheritance, class variables, and instances

class Descriptor:
    def __get__(self, obj, objtype=None):
        return "DESC1"

class Base(object):
    VALUE = "base"
    @classmethod
    def the_value(cls):
        return cls.VALUE # returns "base"

class Sub1(Base):
    THING = "sub1"
    @classmethod
    def the_value(cls):
        return cls.THING # returns "sub1"

# inherits the_value classmethod from Base 
class Sub2(Base):
    VALUE = Descriptor()

b = Base()
# accesses the VALUE class variable via the instance
b.VALUE # => 'base'
# calls the classmethod via the instance
b.the_value() # => 'base

s1 = Sub1()
s1.THING # => 'sub1
s1.the_value() # => 'sub1'

s2 = Sub2()
s2.VALUE # => 'DESC1'
s2.the_value() # => 'DESC1'

# conditionally transforming values in a dict comprehension
transformed = {k: v.upper() if k == "special" else v.lower() for (k, v) in dict.items()}

# Setup logging to write to STDOUT

import logging
import sys

logging.basicConfig(stream=sys.stdout, level=logging.DEBUG)

# Update a local variable from a nested function

def outer():
    val = 1
    def inner():
        nonlocal val
        val += 1
        
# Suppress including the causal chain from an exception raised from an except block

try:
    ...
except SomeException:
    # note: the causal exception is still included in the exception that is raised, 
    # but __suppress_context__ is True, which tells exception handlers to ignore it  
    raise OtherException("message") from None
    
# Function decorators are composed in the order that they are applied

class InnerException(Exception):
      pass

class OuterException(Exception):
      pass

def innerToOuter(function):
    def fn(*args, **kwargs):
        try:
            function(*args, **kwargs)
        except InnerException:
            raise OuterException()
    return fn

def handleOuter(function):
    def fn(*args, **kwargs):
        try:
            function(*args, **kwargs)
        except OuterException:
            print("Handled an OuterException")
        except Exception as e:
            print(f"Handled a non-OuterException: {e}")
    return fn

class Foo:
    # works correctly, InnerExceptionis converted to an OuterException and handled
    @classmethod
    @handleOuter
    @innerToOuter
    def perform1(cls):
        raise InnerException()
        
    # does not work correctly, InnerException is caught and handled before it can be converted to an OuterException
    @classmethod
    @innerToOuter
    @handleOuter
    def perform2(cls):
        raise InnerException()
        
# Flatten a List of Lists
# - only works for for Lists of Lists, fails if any elements are scalars
# - does not flatten anything deeper (i.e. a List of Lists of Lists)

list = [[1,2,3], [4,5], [6]]
flattened = [item for sublist in list for item in sublist]