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0

I have two modules:

#module_a

def make_a(cb = None):

 global a

 a = 1 # It takes a very long time to make "a", so it is stored as a global in module_a.

 if cb != None:

 cb()

And,

#module_b

import module_a

def make_b():

 global b

 #In order to make "b", we need module_a.a, however there is no guarantee it has been made.
 if 'a' not in dir(module_a):

 module_a.make_a(make_b)

 return

 else:

 b = module_a.a + 1

make_b()

print(b) # 2

In the above code, the function make_b in module_b makes something named b, which depends on something named a in module_a. In order to satisfy its dependency, it calls make_a with a callback to make_b. Once a is made, then make_b is called again and b is made.

This pattern makes sense to me, but I am wondering if this is a generally accepted approach to doing this or if it is an anti-pattern.

Is there a more canonical approach to satisfying such a dependency in Python i.e., the Pythonic way?

python callback dependencies

share|improve this question

edited Mar 8 at 23:29

adpatter

asked Mar 8 at 23:10

adpatteradpatter

8919

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  I would consider the use of global variables an anti-pattern, yes. The "canonical" way is to provide arguments to your functions and return values from them.
  
  – juanpa.arrivillaga
  Mar 8 at 23:12
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  I cannot do that because it takes a long time to make "a".
  
  – adpatter
  Mar 8 at 23:12
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Sure you can. Global variables are not the usual way of implementing caching to begin with
  
  – juanpa.arrivillaga
  Mar 8 at 23:13
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  If I did that, then it would merely be a getter - I would still need for make_b to execute after a is made.
  
  – adpatter
  Mar 8 at 23:14
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Your current approach looks like a leaky abstraction, because every client that wants to access a must include that ugly attribute-check and callback. A much cleaner approach would be to eliminate the callback and put the attribute-check in module_a. Clients should then call make_a, which would cache and return the required value. This would also eliminate the need for a mutable module attribute, which as others have noted, is also an anti-pattern in python.
  
  – ekhumoro
  Mar 8 at 23:58
  

  
  
  
  

 | 
show 6 more comments

0

I have two modules:

#module_a

def make_a(cb = None):

 global a

 a = 1 # It takes a very long time to make "a", so it is stored as a global in module_a.

 if cb != None:

 cb()

And,

#module_b

import module_a

def make_b():

 global b

 #In order to make "b", we need module_a.a, however there is no guarantee it has been made.
 if 'a' not in dir(module_a):

 module_a.make_a(make_b)

 return

 else:

 b = module_a.a + 1

make_b()

print(b) # 2

In the above code, the function make_b in module_b makes something named b, which depends on something named a in module_a. In order to satisfy its dependency, it calls make_a with a callback to make_b. Once a is made, then make_b is called again and b is made.

This pattern makes sense to me, but I am wondering if this is a generally accepted approach to doing this or if it is an anti-pattern.

Is there a more canonical approach to satisfying such a dependency in Python i.e., the Pythonic way?

python callback dependencies

share|improve this question

edited Mar 8 at 23:29

adpatter

asked Mar 8 at 23:10

adpatteradpatter

8919

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  I would consider the use of global variables an anti-pattern, yes. The "canonical" way is to provide arguments to your functions and return values from them.
  
  – juanpa.arrivillaga
  Mar 8 at 23:12
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  I cannot do that because it takes a long time to make "a".
  
  – adpatter
  Mar 8 at 23:12
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Sure you can. Global variables are not the usual way of implementing caching to begin with
  
  – juanpa.arrivillaga
  Mar 8 at 23:13
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  If I did that, then it would merely be a getter - I would still need for make_b to execute after a is made.
  
  – adpatter
  Mar 8 at 23:14
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Your current approach looks like a leaky abstraction, because every client that wants to access a must include that ugly attribute-check and callback. A much cleaner approach would be to eliminate the callback and put the attribute-check in module_a. Clients should then call make_a, which would cache and return the required value. This would also eliminate the need for a mutable module attribute, which as others have noted, is also an anti-pattern in python.
  
  – ekhumoro
  Mar 8 at 23:58
  

  
  
  
  

 | 
show 6 more comments

I have two modules:

#module_a

def make_a(cb = None):

 global a

 a = 1 # It takes a very long time to make "a", so it is stored as a global in module_a.

 if cb != None:

 cb()

And,

#module_b

import module_a

def make_b():

 global b

 #In order to make "b", we need module_a.a, however there is no guarantee it has been made.
 if 'a' not in dir(module_a):

 module_a.make_a(make_b)

 return

 else:

 b = module_a.a + 1

make_b()

print(b) # 2

In the above code, the function make_b in module_b makes something named b, which depends on something named a in module_a. In order to satisfy its dependency, it calls make_a with a callback to make_b. Once a is made, then make_b is called again and b is made.

This pattern makes sense to me, but I am wondering if this is a generally accepted approach to doing this or if it is an anti-pattern.

Is there a more canonical approach to satisfying such a dependency in Python i.e., the Pythonic way?

python callback dependencies

share|improve this question

edited Mar 8 at 23:29

adpatter

asked Mar 8 at 23:10

adpatteradpatter

8919

#module_a

def make_a(cb = None):

 global a

 a = 1 # It takes a very long time to make "a", so it is stored as a global in module_a.

 if cb != None:

 cb()

#module_b

import module_a

def make_b():

 global b

 #In order to make "b", we need module_a.a, however there is no guarantee it has been made.
 if 'a' not in dir(module_a):

 module_a.make_a(make_b)

 return

 else:

 b = module_a.a + 1

make_b()

print(b) # 2

share|improve this question

edited Mar 8 at 23:29

adpatter

asked Mar 8 at 23:10

adpatteradpatter

8919

share|improve this question

edited Mar 8 at 23:29

adpatter

asked Mar 8 at 23:10

adpatteradpatter

8919

asked Mar 8 at 23:10

adpatteradpatter

8919

asked Mar 8 at 23:10

adpatteradpatter

8919

1 Answer
1

active

oldest

votes

1

To do this in a "Pythonic" way you should introduce an Observer pattern that emits an event once a is computed and b should subscribe to a in order to be called after a is finished.

Here you have an observer example

share|improve this answer

edited Mar 8 at 23:20

Robert Harvey♦

150k35277422

answered Mar 8 at 23:16

Pablo MartinezPablo Martinez

3215

add a comment | 

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1

To do this in a "Pythonic" way you should introduce an Observer pattern that emits an event once a is computed and b should subscribe to a in order to be called after a is finished.

Here you have an observer example

share|improve this answer

edited Mar 8 at 23:20

Robert Harvey♦

150k35277422

answered Mar 8 at 23:16

Pablo MartinezPablo Martinez

3215

add a comment | 

1

To do this in a "Pythonic" way you should introduce an Observer pattern that emits an event once a is computed and b should subscribe to a in order to be called after a is finished.

Here you have an observer example

share|improve this answer

edited Mar 8 at 23:20

Robert Harvey♦

150k35277422

answered Mar 8 at 23:16

Pablo MartinezPablo Martinez

3215

add a comment | 

To do this in a "Pythonic" way you should introduce an Observer pattern that emits an event once a is computed and b should subscribe to a in order to be called after a is finished.

Here you have an observer example

share|improve this answer

edited Mar 8 at 23:20

Robert Harvey♦

150k35277422

answered Mar 8 at 23:16

Pablo MartinezPablo Martinez

3215

share|improve this answer

edited Mar 8 at 23:20

Robert Harvey♦

150k35277422

answered Mar 8 at 23:16

Pablo MartinezPablo Martinez

3215

edited Mar 8 at 23:20

Robert Harvey♦

150k35277422

edited Mar 8 at 23:20

Robert Harvey♦

150k35277422

answered Mar 8 at 23:16

Pablo MartinezPablo Martinez

3215

answered Mar 8 at 23:16

Pablo MartinezPablo Martinez

3215