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1

Is it possible, that a lambda function can return a False value, when given to the bool function?

This lambda function for example yields True:

bool(lambda x:[])
True

python python-3.x

share|improve this question

edited Mar 28 at 10:11

cs95

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asked Mar 28 at 8:53

DavidDavid

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• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Lambdas are just an alternative way to write functions. You can create a class that behaves as a function and is itself falsey, but it would not be a lambda function.
  
  – khelwood
  Mar 28 at 8:57
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  So functions are always evaluated as true?
  
  – David
  Mar 28 at 9:01
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Functions are truthy by default, like nearly everything. You can create something customised, but it wouldn't be a lambda function.
  
  – khelwood
  Mar 28 at 9:04
  

  
  
  
  

add a comment | 

1

Is it possible, that a lambda function can return a False value, when given to the bool function?

This lambda function for example yields True:

bool(lambda x:[])
True

python python-3.x

share|improve this question

edited Mar 28 at 10:11

cs95

166k3232 gold badges240240 silver badges303303 bronze badges

asked Mar 28 at 8:53

DavidDavid

54633 silver badges1212 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Lambdas are just an alternative way to write functions. You can create a class that behaves as a function and is itself falsey, but it would not be a lambda function.
  
  – khelwood
  Mar 28 at 8:57
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  So functions are always evaluated as true?
  
  – David
  Mar 28 at 9:01
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Functions are truthy by default, like nearly everything. You can create something customised, but it wouldn't be a lambda function.
  
  – khelwood
  Mar 28 at 9:04
  

  
  
  
  

add a comment | 

Is it possible, that a lambda function can return a False value, when given to the bool function?

This lambda function for example yields True:

bool(lambda x:[])
True

python python-3.x

share|improve this question

edited Mar 28 at 10:11

cs95

166k3232 gold badges240240 silver badges303303 bronze badges

asked Mar 28 at 8:53

DavidDavid

54633 silver badges1212 bronze badges

bool(lambda x:[])
True

share|improve this question

edited Mar 28 at 10:11

cs95

166k3232 gold badges240240 silver badges303303 bronze badges

asked Mar 28 at 8:53

DavidDavid

54633 silver badges1212 bronze badges

share|improve this question

edited Mar 28 at 10:11

cs95

166k3232 gold badges240240 silver badges303303 bronze badges

asked Mar 28 at 8:53

DavidDavid

54633 silver badges1212 bronze badges

edited Mar 28 at 10:11

cs95

166k3232 gold badges240240 silver badges303303 bronze badges

edited Mar 28 at 10:11

cs95

166k3232 gold badges240240 silver badges303303 bronze badges

asked Mar 28 at 8:53

DavidDavid

54633 silver badges1212 bronze badges

asked Mar 28 at 8:53

DavidDavid

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3 Answers
3

active

oldest

votes

1

Only the objects specified in Truth Value Testing in the docs, as well as objects whose __bool__ or __len__ methods return False or 0 respectively, are falsy objects.

Everything else (yes, this includes any lambda) is truthy.

share|improve this answer

edited Jun 19 at 6:54

answered Mar 28 at 9:02

cs95cs95

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add a comment | 

3

no, you can not do that with pure lambda expressions.

lambda x:[]

is of the type

<class 'function'>

and as the documentation says, there is nothing of that type that will turn out to be falsy - so it will come out truthy; i.e. passing it to bool will return True.

if you want a funtion (a callable) that evaluates to False i would exactly do what is described in khelwood's answer.

share|improve this answer

edited Mar 28 at 9:20

answered Mar 28 at 8:55

hiro protagonisthiro protagonist

27k88 gold badges5151 silver badges7474 bronze badges

add a comment | 

2

Lambdas are just an alternative way to write functions. Like nearly everything, they are truthy by default. You can create a class whose instances behave as functions and are themselves falsey. They wouldn't be lambda functions, but there's no reason that should be a problem.

For instance:

class FalseyFunction:
 def __init__(self, func):
 self.func = func
 def __call__(self, *args, **kwargs):
 return self.func(*args, **kwargs)
 def __bool__(self):
 return False

>>> f = FalseyFunction(lambda x:[])
>>> f(0)
[]
>>> bool(f)
False

share|improve this answer

answered Mar 28 at 9:03

khelwoodkhelwood

34.9k77 gold badges4545 silver badges6868 bronze badges

add a comment | 

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1

Only the objects specified in Truth Value Testing in the docs, as well as objects whose __bool__ or __len__ methods return False or 0 respectively, are falsy objects.

Everything else (yes, this includes any lambda) is truthy.

share|improve this answer

edited Jun 19 at 6:54

answered Mar 28 at 9:02

cs95cs95

166k3232 gold badges240240 silver badges303303 bronze badges

add a comment | 

1

Only the objects specified in Truth Value Testing in the docs, as well as objects whose __bool__ or __len__ methods return False or 0 respectively, are falsy objects.

Everything else (yes, this includes any lambda) is truthy.

share|improve this answer

edited Jun 19 at 6:54

answered Mar 28 at 9:02

cs95cs95

166k3232 gold badges240240 silver badges303303 bronze badges

add a comment | 

Only the objects specified in Truth Value Testing in the docs, as well as objects whose __bool__ or __len__ methods return False or 0 respectively, are falsy objects.

Everything else (yes, this includes any lambda) is truthy.

share|improve this answer

edited Jun 19 at 6:54

answered Mar 28 at 9:02

cs95cs95

166k3232 gold badges240240 silver badges303303 bronze badges

share|improve this answer

edited Jun 19 at 6:54

answered Mar 28 at 9:02

cs95cs95

166k3232 gold badges240240 silver badges303303 bronze badges

answered Mar 28 at 9:02

cs95cs95

166k3232 gold badges240240 silver badges303303 bronze badges

answered Mar 28 at 9:02

cs95cs95

166k3232 gold badges240240 silver badges303303 bronze badges

3

no, you can not do that with pure lambda expressions.

lambda x:[]

is of the type

<class 'function'>

and as the documentation says, there is nothing of that type that will turn out to be falsy - so it will come out truthy; i.e. passing it to bool will return True.

if you want a funtion (a callable) that evaluates to False i would exactly do what is described in khelwood's answer.

share|improve this answer

edited Mar 28 at 9:20

answered Mar 28 at 8:55

hiro protagonisthiro protagonist

27k88 gold badges5151 silver badges7474 bronze badges

add a comment | 

3

no, you can not do that with pure lambda expressions.

lambda x:[]

is of the type

<class 'function'>

and as the documentation says, there is nothing of that type that will turn out to be falsy - so it will come out truthy; i.e. passing it to bool will return True.

if you want a funtion (a callable) that evaluates to False i would exactly do what is described in khelwood's answer.

share|improve this answer

edited Mar 28 at 9:20

answered Mar 28 at 8:55

hiro protagonisthiro protagonist

27k88 gold badges5151 silver badges7474 bronze badges

add a comment | 

no, you can not do that with pure lambda expressions.

lambda x:[]

is of the type

<class 'function'>

and as the documentation says, there is nothing of that type that will turn out to be falsy - so it will come out truthy; i.e. passing it to bool will return True.

if you want a funtion (a callable) that evaluates to False i would exactly do what is described in khelwood's answer.

share|improve this answer

edited Mar 28 at 9:20

answered Mar 28 at 8:55

hiro protagonisthiro protagonist

27k88 gold badges5151 silver badges7474 bronze badges

lambda x:[]

<class 'function'>

share|improve this answer

edited Mar 28 at 9:20

answered Mar 28 at 8:55

hiro protagonisthiro protagonist

27k88 gold badges5151 silver badges7474 bronze badges

answered Mar 28 at 8:55

hiro protagonisthiro protagonist

27k88 gold badges5151 silver badges7474 bronze badges

answered Mar 28 at 8:55

hiro protagonisthiro protagonist

27k88 gold badges5151 silver badges7474 bronze badges

2

Lambdas are just an alternative way to write functions. Like nearly everything, they are truthy by default. You can create a class whose instances behave as functions and are themselves falsey. They wouldn't be lambda functions, but there's no reason that should be a problem.

For instance:

class FalseyFunction:
 def __init__(self, func):
 self.func = func
 def __call__(self, *args, **kwargs):
 return self.func(*args, **kwargs)
 def __bool__(self):
 return False

>>> f = FalseyFunction(lambda x:[])
>>> f(0)
[]
>>> bool(f)
False

share|improve this answer

answered Mar 28 at 9:03

khelwoodkhelwood

34.9k77 gold badges4545 silver badges6868 bronze badges

add a comment | 

2

Lambdas are just an alternative way to write functions. Like nearly everything, they are truthy by default. You can create a class whose instances behave as functions and are themselves falsey. They wouldn't be lambda functions, but there's no reason that should be a problem.

For instance:

class FalseyFunction:
 def __init__(self, func):
 self.func = func
 def __call__(self, *args, **kwargs):
 return self.func(*args, **kwargs)
 def __bool__(self):
 return False

>>> f = FalseyFunction(lambda x:[])
>>> f(0)
[]
>>> bool(f)
False

share|improve this answer

answered Mar 28 at 9:03

khelwoodkhelwood

34.9k77 gold badges4545 silver badges6868 bronze badges

add a comment | 

Lambdas are just an alternative way to write functions. Like nearly everything, they are truthy by default. You can create a class whose instances behave as functions and are themselves falsey. They wouldn't be lambda functions, but there's no reason that should be a problem.

For instance:

class FalseyFunction:
 def __init__(self, func):
 self.func = func
 def __call__(self, *args, **kwargs):
 return self.func(*args, **kwargs)
 def __bool__(self):
 return False

>>> f = FalseyFunction(lambda x:[])
>>> f(0)
[]
>>> bool(f)
False

share|improve this answer

answered Mar 28 at 9:03

khelwoodkhelwood

34.9k77 gold badges4545 silver badges6868 bronze badges

class FalseyFunction:
 def __init__(self, func):
 self.func = func
 def __call__(self, *args, **kwargs):
 return self.func(*args, **kwargs)
 def __bool__(self):
 return False

>>> f = FalseyFunction(lambda x:[])
>>> f(0)
[]
>>> bool(f)
False

share|improve this answer

answered Mar 28 at 9:03

khelwoodkhelwood

34.9k77 gold badges4545 silver badges6868 bronze badges

answered Mar 28 at 9:03

khelwoodkhelwood

34.9k77 gold badges4545 silver badges6868 bronze badges

answered Mar 28 at 9:03

khelwoodkhelwood

34.9k77 gold badges4545 silver badges6868 bronze badges