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One of the usecases I have found very useful is to do a `raise NotImplementedError()` _inside_ the child method of an `@abstractmethod`-decorated base class method. – Yes, it's a mouthful but what it **really means** is **graceful simplicity**.
----------
# Example
Imagine writing a control script for a family of electronic measurement modules (i.e. physical devices).
The functionality of each module is narrowly-defined, implementing just one dedicated function:
one could be an array of relays, another a multi-channel DAC or ADC, another an ammeter etc.
Many of the low-level commands would be shared between the modules for example to read
their ID numbers or to send a command to them. Let's see what we have at this point:
## Base Class
from abc import ABC, abstractmethod #< we'll make use of these later
class Generic(ABC):
''' Base class for all measurement modules. '''
# Shared functions
def __init__(self):
# do what you must...
def _read_ID(self):
# same for all the modules
def _send_command(self, value):
# same for all the modules
----------
### Shared Verbs
We then realise that much of the module-specific command verbs and, therefore, the
logic of their interfaces is also shared. Here are 3 different verbs whose meaning
would be self-explanatory considering a number of target modules.
**`get(channel)`:**
- **relay:** get the on/off status of the relay on `channel`
- **DAC:** get the _output_ voltage on `channel`
- **ADC:** get the _input_ voltage on `channel`
**`enable(channel)`:**
- **relay:** enable the use of the relay on `channel`
- **DAC:** enable the use of the _output_ channel on `channel`
- **ADC:** enable the use of the _input_ channel on `channel`
**`set(channel)`:**
- **relay:** set the relay on `channel` on/off
- **DAC:** set the _output_ voltage on `channel`
- **ADC:** hmm... nothing _logical_ comes to mind.
----------
### Shared Verbs Become Enforced Verbs
I'd argue that there is a strong case for the above verbs to be shared across the modules
as we saw that their meaning is evident for each one of them. I'd continue writing my
base class `Generic` like so:
class Generic(ABC): # ...continued
@abstractmethod
def get(self, channel):
pass
@abstractmethod
def enable(self, channel):
pass
@abstractmethod
def set(self, channel):
pass
----------
## Subclasses
We now know that our subclasses will all have to define these methods. Let's see what it
could look like for the ADC module:
class ADC(Generic):
def __init__(self):
super().__init__() #< applies to all modules
# more init code specific to the ADC module
def get(self, channel):
# returns the input value measured on the given 'channel'
def enable(self, channel):
# enables accessing the given 'channel'
You may now be wondering:
> But this won't work for the **ADC** module as **`set`** makes no sense there!
You're right: NOT implementing `set` is not an option as Python would then fire the error below at you
as soon as your as you tried instantiating your ADC object.
~~TypeError: Can't instantiate abstract class 'ADC' with abstract methods 'set'~~
So you must implement `set`, because we made it an _enforced verb_ (aka `@abstractmethod`),
which is shared by two other modules. But, what should you implement if `set` does not make sense for this particular module? What makes the interface (for users) and the code-base (for future maintenance) as clean as possible?
~~## NotImplementedError to the Rescue~~
By completing the ADC class like this:
class ADC(Generic): # ...continued
def set(self, channel):
raise NotImplementedError("Can't use 'set' on an ADC!")
You are doing three very good things at once:
1. You are protecting a user from erroneously issuing a command ('set') that is
not (and shouldn't!) be implemented for this module.
2. You are telling them **explicitly** what the problem is (see TemporalWolf's link about
'Bare exceptions' for why this is important)
3. You are protecting the implementation of all the other modules for which the _enforced verbs_
do make sense. I.e. you ensure that those modules for which these verbs _do_ make sense will
implement these methods and that **they will do so using exactly these verbs** and not some
other ad-hoc names.

One of the usecases I have found very useful is to do a `raise NotImplementedError()` _inside_ the child method of an `@abstractmethod`-decorated base class method. Yes, it's a mouthful but what it **really means** is **graceful simplicity**.
----------
# Example
Imagine writing a control script for a family of electronic measurement modules (i.e. physical devices).
The functionality of each module is narrowly-defined, implementing just one dedicated function:
one could be an array of relays, another a multi-channel DAC or ADC, another an ammeter etc.
Many of the low-level commands would be shared between the modules for example to read
their ID numbers or to send a command to them. Let's see what we have at this point:
## Base Class
from abc import ABC, abstractmethod #< we'll make use of these later
class Generic(ABC):
''' Base class for all measurement modules. '''
# Shared functions
def __init__(self):
# do what you must...
def _read_ID(self):
# same for all the modules
def _send_command(self, value):
# same for all the modules
----------
### Shared Verbs
We then realise that much of the module-specific command verbs and, therefore, the
logic of their interfaces is also shared. Here are 3 different verbs whose meaning
would be self-explanatory considering a number of target modules.
**`get(channel)`:**
- **relay:** get the on/off status of the relay on `channel`
- **DAC:** get the _output_ voltage on `channel`
- **ADC:** get the _input_ voltage on `channel`
**`enable(channel)`:**
- **relay:** enable the use of the relay on `channel`
- **DAC:** enable the use of the _output_ channel on `channel`
- **ADC:** enable the use of the _input_ channel on `channel`
**`set(channel)`:**
- **relay:** set the relay on `channel` on/off
- **DAC:** set the _output_ voltage on `channel`
- **ADC:** hmm... nothing _logical_ comes to mind.
----------
### Shared Verbs Become Enforced Verbs
I'd argue that there is a strong case for the above verbs to be shared across the modules
as we saw that their meaning is evident for each one of them. I'd continue writing my
base class `Generic` like so:
class Generic(ABC): # ...continued
@abstractmethod
def get(self, channel):
pass
@abstractmethod
def enable(self, channel):
pass
@abstractmethod
def set(self, channel):
pass
----------
## Subclasses
We now know that our subclasses will all have to define these methods. Let's see what it
could look like for the ADC module:
class ADC(Generic):
def __init__(self):
super().__init__() #< applies to all modules
# more init code specific to the ADC module
def get(self, channel):
# returns the input value measured on the given 'channel'
def enable(self, channel):
# enables accessing the given 'channel'
You may now be wondering:
> But this won't work for the **ADC** module as **`set`** makes no sense there!
You're right: NOT implementing `set` is not an option as Python would then fire the error below at you
as soon as your as you tried instantiating your ADC object.
```none
TypeError: Can't instantiate abstract class 'ADC' with abstract methods 'set'
```
So you must implement `set`, because we made it an _enforced verb_ (aka `@abstractmethod`),
which is shared by two other modules. But, what should you implement if `set` does not make sense for this particular module? What makes the interface (for users) and the code-base (for future maintenance) as clean as possible?
## `NotImplementedError` to the Rescue
By completing the ADC class like this:
class ADC(Generic): # ...continued
def set(self, channel):
raise NotImplementedError("Can't use 'set' on an ADC!")
You are doing three very good things at once:
1. You are protecting a user from erroneously issuing a command ('set') that is
not (and shouldn't!) be implemented for this module.
2. You are telling them **explicitly** what the problem is (see TemporalWolf's link about
'Bare exceptions' for why this is important)
3. You are protecting the implementation of all the other modules for which the _enforced verbs_
do make sense. I.e. you ensure that those modules for which these verbs _do_ make sense will
implement these methods and that **they will do so using exactly these verbs** and not some
other ad-hoc names.

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