When is del useful in python?

Question

I can\'t really think of any reason why python needs the del keyword (and most languages seem to not have a similar keyword). For instance, rather than deletin

Answer 1

To add a few points to above answers: del x

Definition of x indicates r -> o (a reference r pointing to an object o) but del x changes r rather than o. It is an operation on the reference (pointer) to object rather than the object associated with x. Distinguishing between r and o is key here.

• It removes it from locals().
• Removes it from globals() if x belongs there.
• Removes it from the stack frame (removes the reference physically from it, but the object itself resides in object pool and not in the stack frame).
• Removes it from the current scope. It is very useful to limit the span of definition of a local variable, which otherwise can cause problems.
• It is more about declaration of the name rather than definition of content.
• It affects where x belongs to, not where x points to. The only physical change in memory is this. For example if x is in a dictionary or list, it (as a reference) is removed from there(and not necessarily from the object pool). In this example, the dictionary it belongs is the stack frame (locals()), which overlaps with globals().

Answer 2

When is del useful in python?

You can use it to remove a single element of an array instead of the slice syntax x[i:i+1]=[]. This may be useful if for example you are in os.walk and wish to delete an element in the directory. I would not consider a keyword useful for this though, since one could just make a [].remove(index) method (the .remove method is actually search-and-remove-first-instance-of-value).

Answer 3

One place I've found del useful is cleaning up extraneous variables in for loops:

for x in some_list:
  do(x)
del x

Now you can be sure that x will be undefined if you use it outside the for loop.

Answer 4

Force closing a file after using numpy.load:

A niche usage perhaps but I found it useful when using numpy.load to read a file. Every once in a while I would update the file and need to copy a file with the same name to the directory.

I used del to release the file and allow me to copy in the new file.

Note I want to avoid the with context manager as I was playing around with plots on the command line and didn't want to be pressing tab a lot!

See this question.

Answer 5

There's this part of what del does (from the Python Language Reference):

Deletion of a name removes the binding of that name from the local or global namespace

Assigning None to a name does not remove the binding of the name from the namespace.

(I suppose there could be some debate about whether removing a name binding is actually useful, but that's another question.)

Answer 6

I would like to elaborate on the accepted answer to highlight the nuance between setting a variable to None versus removing it with del:

Given the variable foo = 'bar', and the following function definition:

def test_var(var):
    if var:
        print('variable tested true')
    else:
        print('variable tested false')

Once initially declared, test_var(foo) yields variable tested true as expected.

Now try:

foo = None
test_var(foo)

which yields variable tested false.

Contrast this behavior with:

del foo
test_var(foo)

which now raises NameError: name 'foo' is not defined.