In Python, is use of `del` statement a code smell?
Question:
I tend to use it whenever I am working on a prototype script, and:
- Use a somewhat common variable (such as
fileCount), and
- Have a large method (20+ lines), and
- Do not use classes or namespaces yet.
In this situation, in order to avoid potential variable clash, I delete the bugger as soon as I am done with it. I know, in a production code I should avoid 1., 2., and 3., but going from a prototype that works to a completely polished class is time consuming. Sometimes I might want to settle for a sub-optimal, quick refactoring job. In that case I find keeping the del statements handy. Am I developing an unnecessary, bad habit? Is del totally avoidable? When would it be a good thing?
Answers:
I don’t think that del by itself is a code smell.
Reusing a variable name in the same namespace is definitely a code smell as is not using classes and other namespaces where appropriate. So using del to facilitate that sort of thing is a code smell.
The only really appropriate use of del that I can think of off the top of my head is breaking cyclic references which are often a code smell as well (and often times, this isn’t even necessary). Remember, all del does is delete the reference to the object and not the object itself. That will be taken care of by either reference counting or garbage collecting.
>>> a = [1, 2]
>>> b = a
>>> del a
>>> a
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'a' is not defined
>>> b
[1, 2]
You can see that the list is kept alive after the del statement because b still holds a reference to it.
So, while del isn’t really a code smell, it can be associated with things that are.
Any code that’s well organized in functions, classes and methods doesn’t need del except in exceptional circumstances. Aim to build your apps well factored from the start by using more functions and methods, avoid reusing variable names, etc.
The use of a del statement is OK – it doesn’t lead to any trouble, I use it often when I use Python as a replacement for shell scripts on my system, and when I’m making script experiments. However, if it appears often in a real application or library, it is an indication that something isn’t all right, probably badly structured code. I never had to use it in an application, and you’d rarely see it used anywhere on code that’s been released.
I tend to use it whenever I am working on a prototype script, and:
- Use a somewhat common variable (such as
fileCount), and - Have a large method (20+ lines), and
- Do not use classes or namespaces yet.
In this situation, in order to avoid potential variable clash, I delete the bugger as soon as I am done with it. I know, in a production code I should avoid 1., 2., and 3., but going from a prototype that works to a completely polished class is time consuming. Sometimes I might want to settle for a sub-optimal, quick refactoring job. In that case I find keeping the del statements handy. Am I developing an unnecessary, bad habit? Is del totally avoidable? When would it be a good thing?
I don’t think that del by itself is a code smell.
Reusing a variable name in the same namespace is definitely a code smell as is not using classes and other namespaces where appropriate. So using del to facilitate that sort of thing is a code smell.
The only really appropriate use of del that I can think of off the top of my head is breaking cyclic references which are often a code smell as well (and often times, this isn’t even necessary). Remember, all del does is delete the reference to the object and not the object itself. That will be taken care of by either reference counting or garbage collecting.
>>> a = [1, 2]
>>> b = a
>>> del a
>>> a
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'a' is not defined
>>> b
[1, 2]
You can see that the list is kept alive after the del statement because b still holds a reference to it.
So, while del isn’t really a code smell, it can be associated with things that are.
Any code that’s well organized in functions, classes and methods doesn’t need del except in exceptional circumstances. Aim to build your apps well factored from the start by using more functions and methods, avoid reusing variable names, etc.
The use of a del statement is OK – it doesn’t lead to any trouble, I use it often when I use Python as a replacement for shell scripts on my system, and when I’m making script experiments. However, if it appears often in a real application or library, it is an indication that something isn’t all right, probably badly structured code. I never had to use it in an application, and you’d rarely see it used anywhere on code that’s been released.