Short-circuit evaluation like Python's "and" while storing results of checks

Question

I have multiple expensive functions that return results. I want to return a tuple of the results of all the checks if all the checks succeed. However, if one check fails I don't want to call the later checks, like the short-circuiting behavior of and. I could nest if statements, but that will get out of hand if there are a lot of checks. How can I get the short-circuit behavior of and while also storing the results for later use?

def check_a():
    # do something and return the result,
    # for simplicity, just make it "A"
    return "A"

def check_b():
    # do something and return the result,
    # for simplicity, just make it "B"
    return "B"

...

This doesn't short-circuit:

a = check_a()
b = check_b()
c = check_c()

if a and b and c:
    return a, b, c

This is messy if there are many checks:

if a:
   b = check_b()

   if b:
      c = check_c()

      if c:
          return a, b, c

Is there a shorter way to do this?

Answer 1

Just use a plain old for loop:

results = {}
for function in [check_a, check_b, ...]:
    results[function.__name__] = result = function()
    if not result:
        break

The results will be a mapping of the function name to their return values, and you can do what you want with the values after the loop breaks.

Use an else clause on the for loop if you want special handling for the case where all of the functions have returned truthy results.

Answer 2

Write a function that takes an iterable of functions to run. Call each one and append the result to a list, or return None if the result is False. Either the function will stop calling further checks after one fails, or it will return the results of all the checks.

def all_or_none(checks, *args, **kwargs):
    out = []

    for check in checks:
        rv = check(*args, **kwargs)

        if not rv:
            return None

        out.append(rv)

    return out

rv = all_or_none((check_a, check_b, check_c))

# rv is a list if all checks passed, otherwise None
if rv is not None:
    return rv

def check_a(obj):
    ...

def check_b(obj):
    ...

# pass arguments to each check, useful for writing reusable checks
rv = all_or_none((check_a, check_b), obj=my_object)

Answer 3

In other languages that did have assignments as expressions you would be able to use

if (a = check_a()) and (b = check_b()) and (c = check_c()):

but Python is no such language. Still, we can circumvent the restriction and emulate that behaviour:

result = []
def put(value):
    result.append(value)
    return value

if put(check_a()) and put(check_b()) and put(check_c()):
    # if you need them as variables, you could do
    # (a, b, c) = result
    # but you just want
    return tuple(result)

This might loosen the connection between the variables and function calls a bit too much, so if you want to do lots of separate things with the variables, instead of using the result elements in the order they were put in the list, I would rather avoid this approach. Still, it might be quicker and shorter than some loop.

Answer 4

You could use either a list or an OrderedDict, using a for loop would serve the purpose of emulating short circuiting.

from collections import OrderedDict


def check_a():
    return "A"


def check_b():
    return "B"


def check_c():
    return "C"


def check_d():
    return False


def method1(*args):
    results = []
    for i, f in enumerate(args):
        value = f()
        results.append(value)
        if not value:
            return None

    return results


def method2(*args):
    results = OrderedDict()

    for f in args:
        results[f.__name__] = result = f()
        if not result:
            return None

    return results

# Case 1, it should return check_a, check_b, check_c
for m in [method1, method2]:
    print(m(check_a, check_b, check_c))

# Case 1, it should return None
for m in [method1, method2]:
    print(m(check_a, check_b, check_d, check_c))

Answer 5

There are lots of ways to do this! Here's another.

You can use a generator expression to defer the execution of the functions. Then you can use itertools.takewhile to implement the short-circuiting logic by consuming items from the generator until one of them is false.

from itertools import takewhile
functions = (check_a, check_b, check_c)
generator = (f() for f in functions)
results = tuple(takewhile(bool, generator))
if len(results) == len(functions):
    return results

Answer 6

Another way to tackle this is using a generator, since generators use lazy evaluation. First put all checks into a generator:

def checks():
    yield check_a()
    yield check_b()
    yield check_c()

Now you could force evaluation of everything by converting it to a list:

list(checks())

But the standard all function does proper short cut evaluation on the iterator returned from checks(), and returns whether all elements are truthy:

all(checks())

Last, if you want the results of succeeding checks up to the failure you can use itertools.takewhile to take the first run of truthy values only. Since the result of takewhile is lazy itself you'll need to convert it to a list to see the result in a REPL:

from itertools import takewhile
takewhile(lambda x: x, checks())
list(takewhile(lambda x: x, checks()))

Answer 7

main logic:

results = list(takewhile(lambda x: x, map(lambda x: x(), function_list)))
if len(results) == len(function_list):
  return results

you can learn a lot about collection transformations if you look at all methods of an api like http://www.scala-lang.org/api/2.11.7/#scala.collection.immutable.List and search/implement python equivalents

logic with setup and alternatives:

import sys
if sys.version_info.major == 2:
  from collections import imap
  map = imap

def test(bool):
  def inner():
    print(bool)
    return bool
  return inner

def function_for_return():
  function_list = [test(True),test(True),test(False),test(True)]

  from itertools import takewhile

  print("results:")

  results = list(takewhile(lambda x:x,map(lambda x:x(),function_list)))
  if len(results) == len(function_list):
    return results

  print(results)
  #personally i prefer another syntax:
  class Iterator(object):
    def __init__(self,iterable):
      self.iterator = iter(iterable)

    def __next__(self):
      return next(self.iterator)

    def __iter__(self):
      return self

    def map(self,f):
      return Iterator(map(f,self.iterator))

    def takewhile(self,f):
      return Iterator(takewhile(f,self.iterator))

  print("results2:")
  results2 = list(
    Iterator(function_list)
      .map(lambda x:x())
      .takewhile(lambda x:x)    
  )

  print(results2)

  print("with additional information")
  function_list2 = [(test(True),"a"),(test(True),"b"),(test(False),"c"),(test(True),"d")]
  results3 = list(
    Iterator(function_list2)
      .map(lambda x:(x[0](),x[1]))
      .takewhile(lambda x:x[0])    
  )
  print(results3)

function_for_return()

Answer 8

Flexible short circuiting is really best done with Exceptions. For a very simple prototype you could even just assert each check result:

try:
    a = check_a()
    assert a
    b = check_b()
    assert b
    c = check_c()
    assert c
    return  a, b, c
except AssertionException as e:
    return None

You should probably raise a custom Exception instead. You could change your check_X functions to raise Exceptions themself, in an arbitrary nested way. Or you could wrap or decorate your check_X functions to raise errors on falsy return values.

In short, exception handling is very flexible and exactly what you are looking for, don't be afraid to use it. If you learned somewhere that exception handling is not to be used for your own flow control, this does not apply to python. Liberal use of exception handling is considered pythonic, as in EAFP.

Answer 9

If you don't need to take an arbitrary number of expressions at runtime (possibly wrapped in lambdas), you can expand your code directly into this pattern:

def f ():
    try:
        return (<a> or jump(),
                <b> or jump(),
                <c> or jump())
    except NonLocalExit:
        return None

Where those definitions apply:

class NonLocalExit(Exception):
    pass

def jump():
    raise NonLocalExit()

Answer 10

You mentioned 'short-circuiting' in your answer, which can be done with the 'or' statement. Top answer basically does the same thing, but in case someone wants to know more about this behaviour you could do this;

class Container(object):
    def __init__(self):
        self.values = []

    def check_and_cache(self, value, checking_function):
        value_true = checking_function(value)
        if value_true:
            self.values.append(value)
            return True

c = Container()
if not c.check_and_cache(a, check_a) or not c.check_and_cache(b, check_b) or not c.check_and_cache(c, check_c):
    print 'done'
return tuple(c.values)

The 'not .. or' setup of the if statements will result in a 'True' if the check fails, so the overall if statement passes without evaluating the remaining values.

Answer 11

Since I can not comment "wim":s answer as guest, I'll just add an extra answer. Since you want a tuple, you should collect the results in a list and then cast to tuple.

def short_eval(*checks):
    result = []
    for check in checks:
        checked = check()
        if not checked:
            break
        result.append(checked)
    return tuple(result)

# Example
wished = short_eval(check_a, check_b, check_c)

Answer 12

You can try use @lazy_function decorator from lazy_python package. Example of usage:

from lazy import lazy_function, strict

@lazy_function
def check(a, b):
    strict(print('Call: {} {}'.format(a, b)))
    if a + b > a * b:
        return '{}, {}'.format(a, b)

a = check(-1, -2)
b = check(1, 2)
c = check(-1, 2)

print('First condition')
if c and a and b: print('Ok: {}'.format((a, b)))

print('Second condition')
if c and b: print('Ok: {}'.format((c, b)))
# Output:
# First condition
# Call: -1 2
# Call: -1 -2
# Second condition
# Call: 1 2
# Ok: ('-1, 2', '1, 2')

Answer 13

This is similar to Bergi's answer but I think that answer misses the point of wanting separate functions (check_a, check_b, check_c):

list1 = []

def check_a():
    condition = True
    a = 1
    if (condition):
        list1.append(a)
        print ("checking a")
        return True
    else:
        return False

def check_b():
    condition = False
    b = 2
    if (condition):
        list1.append(b)
        print ("checking b")
        return True
    else:
        return False

def check_c():
    condition = True
    c = 3
    if (condition):
        list1.append(c)
        print ("checking c")
        return True
    else:
        return False


if check_a() and check_b() and check_c():
    # won't get here

tuple1 = tuple(list1)    
print (tuple1)    

# output is:
# checking a
# (1,)

Or, if you don't want to use the global list, pass a reference of a local list to each of the functions.

Answer 14

If the main objection is

This is messy if there are many checks:

if a:
   b = check_b()

   if b:
      c = check_c()

      if c:
          return a, b, c

A fairly nice pattern is to reverse the condition and return early

if not a:
    return  # None, or some value, or however you want to handle this
b = check_b()
if not b:
    return
c = check_c()
if not c:
    return

# ok, they were all truthy
return a, b, c