Why does list.append evaluate to false in a boolean context?
Question:
Is there a reason being list.append evaluating to false? Or is it just the C convention of returning 0 when successful that comes into play?
>>> u = []
>>> not u.append(6)
True
Answers:
None evaluates to False and in python a function that does not return anything is assumed to have returned None.
If you type:
>> print u.append(6)
None
Tadaaam 🙂
It modifies the list in-place, and returns None. None evaluates to false.
because .append method returns None, therefore not None evaluates to True. Python on error usually raises an error:
>>> a = ()
>>> a.append(5)
Traceback (most recent call last):
File "<pyshell#1>", line 1, in <module>
a.append(5)
AttributeError: 'tuple' object has no attribute 'append'
Actually, it returns None
>>> print u.append(6)
None
>>> print not None
True
>>>
Most Python methods that mutate a container in-place return None — an application of the principle of Command-query separation. (Python’s always reasonably pragmatic about things, so a few mutators do return a usable value when getting it otherwise would be expensive or a mess — the pop method is a good example of this pragmatism — but those are definitely the exception, not the rule, and there’s no reason to make append an exception).
The list.append function returns None. It just adds the value to the list you are calling the method from.
Here is something that’ll make things clearer:
>>> u = []
>>> not u
False
>>> print(u.append(6)) # u.append(6) == None
None
>>> not u.append(6) # not None == True
True
Method append modifies the list in-place and the return value None
In your case, you are creating an array — [6] — on the fly, then discarding it. The variable b ends up with the return value of None.
Why?
This comply with the principle of Command–query separation devised by Bertrand Meyer.
It states that every method should either be a command that performs an action, or a query that returns data to the caller, but not both.
In your example:
u.append(6)
append modified the state of [], so it’s not a best practice to return a value compliance with the principle.
In theoretical terms, this establishes a measure of sanity, whereby one can reason about a program’s state without simultaneously modifying that state.
CQS is well-suited to the object-oriented methodology such as python.
Is there a reason being list.append evaluating to false? Or is it just the C convention of returning 0 when successful that comes into play?
>>> u = []
>>> not u.append(6)
True
None evaluates to False and in python a function that does not return anything is assumed to have returned None.
If you type:
>> print u.append(6)
None
Tadaaam 🙂
It modifies the list in-place, and returns None. None evaluates to false.
because .append method returns None, therefore not None evaluates to True. Python on error usually raises an error:
>>> a = ()
>>> a.append(5)
Traceback (most recent call last):
File "<pyshell#1>", line 1, in <module>
a.append(5)
AttributeError: 'tuple' object has no attribute 'append'
Actually, it returns None
>>> print u.append(6)
None
>>> print not None
True
>>>
Most Python methods that mutate a container in-place return None — an application of the principle of Command-query separation. (Python’s always reasonably pragmatic about things, so a few mutators do return a usable value when getting it otherwise would be expensive or a mess — the pop method is a good example of this pragmatism — but those are definitely the exception, not the rule, and there’s no reason to make append an exception).
The list.append function returns None. It just adds the value to the list you are calling the method from.
Here is something that’ll make things clearer:
>>> u = []
>>> not u
False
>>> print(u.append(6)) # u.append(6) == None
None
>>> not u.append(6) # not None == True
True
Method append modifies the list in-place and the return value None
In your case, you are creating an array — [6] — on the fly, then discarding it. The variable b ends up with the return value of None.
Why?
This comply with the principle of Command–query separation devised by Bertrand Meyer.
It states that every method should either be a command that performs an action, or a query that returns data to the caller, but not both.
In your example:
u.append(6)
append modified the state of [], so it’s not a best practice to return a value compliance with the principle.
In theoretical terms, this establishes a measure of sanity, whereby one can reason about a program’s state without simultaneously modifying that state.
CQS is well-suited to the object-oriented methodology such as python.