[issue39083] Dictionary get(key, default-expression) not short circuit behavior

[Eric V. Smith]

Eric V. Smith  added the comment:

.get() is just a regular function call. And like all python functions, all of 
the arguments are evaluated before the function is called. There is no 
mechanism in python to delay the evaluation of a arguments.

You might want to look at collections.defaultdict. You can supply a factory 
function, so that the call is delayed until a missing key is found.

--
nosy: +eric.smith
resolution:  -> not a bug
stage:  -> resolved
status: open -> closed

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[issue39083] Dictionary get(key, default-expression) not short circuit behavior

[Martin Meo]

New submission from Martin Meo :

"""
Unexpected behavior report

Dictionary get(key, default-expression) not short circuit behavior

MacOS 10.14.6
sys.version_info(major=3, minor=6, micro=5, releaselevel='final', serial=0)

BACKGROUND
A python dictionary is a data structure that associates a set of keys with a 
set of values.
Accessing a non-existent key produces a KeyError.
Dictionaries have a get() method.
get(key[, default-expression])
Return the value for key if key is in the dictionary, else default-expression.
If default-expression is not given, it defaults to None, so that this method 
never raises a KeyError.

EXPECTED BEHAVIOR
get() would only evaluate default-expression if it has to, when key is not 
found.  It would have short-circuit behavior like boolean operators.

ACTUAL BEHAVIOR
The default-expression DOES get evaluated even when the key IS found in the 
dictionary. And if default-expression is a function call, the function DOES get 
called.

"""

denominations = {0:'zero', 1:'one', 2:'two', 3:'three', 4:'four'}

def foo(n):
print('FOO CALLED. n =', n)
return str(n*10)

words = []
words.append(denominations[1])
words.append(denominations[4])
words.append(denominations.get(1))
words.append(denominations.get(4))
words.append(denominations.get(1, 'ERROR-A'))
words.append(denominations.get(4, 'ERROR-B'))

words.append(denominations.get(22, 'ERROR-1'))
words.append(denominations.get(88, 'ERROR-2'))

words.append(denominations.get(1, foo(1)))
words.append(denominations.get(4, foo(4)))

print(words)



def isItZero(n):
print('ISITZERO CALLED.  n=', n)
return False

a = (True or isItZero(9))   # (True or x) is always True so x is not evaluated

--
messages: 358602
nosy: martinmeo
priority: normal
severity: normal
status: open
title: Dictionary get(key, default-expression) not short circuit behavior
type: behavior
versions: Python 3.7

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