Using the Python walrus operator

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A pattern I often encounter is iterating through a range of values, computing an operation on those values and assigning them to a variable. For example, imagine you are saving files and you don't want to over write them, I create an index and keep increasing it until I get a filename which is available. Something like this:

filename = 'data_{}.dat'
i = 0
while os.path.exists(filename.format(i)):

file = filename.format(i)

Python (I believe 3.8) has introduced a new operator called Walrus , which greatly simplifies the code above:

i = 0
filename = 'data_{}.dat'
while os.path.exists(file := filename.format(i)):
    i += 1

The walrus operator allows to assign variables to expressions within another expression. In this case, we assigned filename.format(i) to file and check its existence.

If you want to see a real-world example of when I would use the walrus, check this few lines of code or these ones , in which I search for an appropriate filename.

A caveat : parenthesis play an important role in order resolution:

var = [1, 2, 3, 4]
if n := len(var) > 3:

Outputs True , while:

var = [1, 2, 3, 4]
if (n := len(var)) > 3:

Outputs 4 . In the example with the filename it was not an issue because I was using already another function with the output of the walrus.

I am still hesitant about adding this syntax, since it only works on latest versions of Python which are still not the default on Linux.


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Aquiles Carattino
Aquiles Carattino
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