Cannot mutate default parameter list of derived class
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The is a minimized script I have:
import random
def genvalue():
return random.randint(1, 100)
class A(object):
def __init__(self, x = genvalue()):
self.x = x
class B(A):
def __init__(self):
super(B, self).__init__()
t1 = A(10)
t2 = B()
t3 = B()
print t1.x
print t2.x
print t3.x
The expected result I wanted is t1.x having the value 10, and the other two having random values, but instead both t2 and t3 have the same values, it is like the genfunc is only called once. I would like to have it called each time when an instance is initiated. Is it possible to do it without messing with the function signatures?
python python-2.7
asked Nov 22 '18 at 11:34
user9613405user9613405
184
add a comment |
The is a minimized script I have:
import random
def genvalue():
return random.randint(1, 100)
class A(object):
def __init__(self, x = genvalue()):
self.x = x
class B(A):
def __init__(self):
super(B, self).__init__()
t1 = A(10)
t2 = B()
t3 = B()
print t1.x
print t2.x
print t3.x
The expected result I wanted is t1.x having the value 10, and the other two having random values, but instead both t2 and t3 have the same values, it is like the genfunc is only called once. I would like to have it called each time when an instance is initiated. Is it possible to do it without messing with the function signatures?
python python-2.7
asked Nov 22 '18 at 11:34
user9613405user9613405
184
add a comment |
The is a minimized script I have:
import random
def genvalue():
return random.randint(1, 100)
class A(object):
def __init__(self, x = genvalue()):
self.x = x
class B(A):
def __init__(self):
super(B, self).__init__()
t1 = A(10)
t2 = B()
t3 = B()
print t1.x
print t2.x
print t3.x
The expected result I wanted is t1.x having the value 10, and the other two having random values, but instead both t2 and t3 have the same values, it is like the genfunc is only called once. I would like to have it called each time when an instance is initiated. Is it possible to do it without messing with the function signatures?
python python-2.7
asked Nov 22 '18 at 11:34
user9613405user9613405
184
The is a minimized script I have:
import random
def genvalue():
return random.randint(1, 100)
class A(object):
def __init__(self, x = genvalue()):
self.x = x
class B(A):
def __init__(self):
super(B, self).__init__()
t1 = A(10)
t2 = B()
t3 = B()
print t1.x
print t2.x
print t3.x
The expected result I wanted is t1.x having the value 10, and the other two having random values, but instead both t2 and t3 have the same values, it is like the genfunc is only called once. I would like to have it called each time when an instance is initiated. Is it possible to do it without messing with the function signatures?
python python-2.7
python python-2.7
asked Nov 22 '18 at 11:34
user9613405user9613405
184
asked Nov 22 '18 at 11:34
user9613405user9613405
184
asked Nov 22 '18 at 11:34
user9613405user9613405
184
asked Nov 22 '18 at 11:34
user9613405user9613405
184
asked Nov 22 '18 at 11:34
user9613405user9613405
184
184
add a comment |
add a comment |
2 Answers
2
active
oldest
votes
genfunc is only called once - when the class description is read for the first time. The parameter is evaluated there - it's not evaluated each time the class is created.
Instead set the default value to None, and if a value isn't given, generated it in your __init__ method instead.
class A(object):
def __init__(self, x=None):
if x is None:
x = genvalue()
self.x = x
answered Nov 22 '18 at 11:37
MatsLindhMatsLindh
26.3k32543
add a comment |
Default arguments are evaluated at callable creation time.
Currently, genvalue is called exactly once in your program, at the time the method __init__ is being being built in order to bind the default value of x to the method.
Demo:
import random
def genvalue():
print('genvalue called')
return random.randint(1, 100)
class A(object):
def __init__(self, x=genvalue()):
self.x = x
print('creating some instances...')
A()
A()
A()
print(A.__init__.__defaults__)
Output:
genvalue called
creating some instances...
(32,)
Use
class A(object):
def __init__(self, x=None):
self.x = x if x is not None else genvalue()
answered Nov 22 '18 at 11:36
timgebtimgeb
51.5k126795
add a comment |
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2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
genfunc is only called once - when the class description is read for the first time. The parameter is evaluated there - it's not evaluated each time the class is created.
Instead set the default value to None, and if a value isn't given, generated it in your __init__ method instead.
class A(object):
def __init__(self, x=None):
if x is None:
x = genvalue()
self.x = x
answered Nov 22 '18 at 11:37
MatsLindhMatsLindh
26.3k32543
add a comment |
genfunc is only called once - when the class description is read for the first time. The parameter is evaluated there - it's not evaluated each time the class is created.
Instead set the default value to None, and if a value isn't given, generated it in your __init__ method instead.
class A(object):
def __init__(self, x=None):
if x is None:
x = genvalue()
self.x = x
answered Nov 22 '18 at 11:37
MatsLindhMatsLindh
26.3k32543
add a comment |
genfunc is only called once - when the class description is read for the first time. The parameter is evaluated there - it's not evaluated each time the class is created.
Instead set the default value to None, and if a value isn't given, generated it in your __init__ method instead.
class A(object):
def __init__(self, x=None):
if x is None:
x = genvalue()
self.x = x
answered Nov 22 '18 at 11:37
MatsLindhMatsLindh
26.3k32543
genfunc is only called once - when the class description is read for the first time. The parameter is evaluated there - it's not evaluated each time the class is created.
Instead set the default value to None, and if a value isn't given, generated it in your __init__ method instead.
class A(object):
def __init__(self, x=None):
if x is None:
x = genvalue()
self.x = x
answered Nov 22 '18 at 11:37
MatsLindhMatsLindh
26.3k32543
answered Nov 22 '18 at 11:37
MatsLindhMatsLindh
26.3k32543
answered Nov 22 '18 at 11:37
MatsLindhMatsLindh
26.3k32543
answered Nov 22 '18 at 11:37
MatsLindhMatsLindh
26.3k32543
26.3k32543
add a comment |
add a comment |
Default arguments are evaluated at callable creation time.
Currently, genvalue is called exactly once in your program, at the time the method __init__ is being being built in order to bind the default value of x to the method.
Demo:
import random
def genvalue():
print('genvalue called')
return random.randint(1, 100)
class A(object):
def __init__(self, x=genvalue()):
self.x = x
print('creating some instances...')
A()
A()
A()
print(A.__init__.__defaults__)
Output:
genvalue called
creating some instances...
(32,)
Use
class A(object):
def __init__(self, x=None):
self.x = x if x is not None else genvalue()
answered Nov 22 '18 at 11:36
timgebtimgeb
51.5k126795
add a comment |
Default arguments are evaluated at callable creation time.
Currently, genvalue is called exactly once in your program, at the time the method __init__ is being being built in order to bind the default value of x to the method.
Demo:
import random
def genvalue():
print('genvalue called')
return random.randint(1, 100)
class A(object):
def __init__(self, x=genvalue()):
self.x = x
print('creating some instances...')
A()
A()
A()
print(A.__init__.__defaults__)
Output:
genvalue called
creating some instances...
(32,)
Use
class A(object):
def __init__(self, x=None):
self.x = x if x is not None else genvalue()
answered Nov 22 '18 at 11:36
timgebtimgeb
51.5k126795
add a comment |
Default arguments are evaluated at callable creation time.
Currently, genvalue is called exactly once in your program, at the time the method __init__ is being being built in order to bind the default value of x to the method.
Demo:
import random
def genvalue():
print('genvalue called')
return random.randint(1, 100)
class A(object):
def __init__(self, x=genvalue()):
self.x = x
print('creating some instances...')
A()
A()
A()
print(A.__init__.__defaults__)
Output:
genvalue called
creating some instances...
(32,)
Use
class A(object):
def __init__(self, x=None):
self.x = x if x is not None else genvalue()
answered Nov 22 '18 at 11:36
timgebtimgeb
51.5k126795
Default arguments are evaluated at callable creation time.
Currently, genvalue is called exactly once in your program, at the time the method __init__ is being being built in order to bind the default value of x to the method.
Demo:
import random
def genvalue():
print('genvalue called')
return random.randint(1, 100)
class A(object):
def __init__(self, x=genvalue()):
self.x = x
print('creating some instances...')
A()
A()
A()
print(A.__init__.__defaults__)
Output:
genvalue called
creating some instances...
(32,)
Use
class A(object):
def __init__(self, x=None):
self.x = x if x is not None else genvalue()
answered Nov 22 '18 at 11:36
timgebtimgeb
51.5k126795
edited Nov 22 '18 at 11:51
answered Nov 22 '18 at 11:36
timgebtimgeb
51.5k126795
answered Nov 22 '18 at 11:36
timgebtimgeb
51.5k126795
answered Nov 22 '18 at 11:36
timgebtimgeb
51.5k126795
51.5k126795
add a comment |
add a comment |
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