Python: Walrus operator

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Python 3.8 added a construct called the "walrus operator". This operator, ":=" has some controversy but also some times when it can be useful.

The Python walrus operator ":=" is used inside an expression to assign a subexpression a value - just as in an assignment statement.

Performing a destructive update of a variable in an expression evaluation is usually considered bad programming practice. Correctness rules assume this does not happen. Avoid this usage.

Using an assignment in an expression can be useful when the variable there is no destructive update. That is, the variable did not previously have a value used in the program.

Note: In a loop where each loop body is distinct from the rest of the code, reassigning a variable that was only used during the last loop body iteration does not violate this principle.

Here is a somewhat contrived example to show the concept.

Here is the Python code [#1]

testList1 = [
	{ "color": "red", "hex":"FF0000"},
	{ "color": "green", "hex":"00FF00"},
	{ "color": "blue", "hex":"0000FF"},
	]
findColor1 = "green"
len1 = len(testList1)
if len1 != 0:
	for colorDict1 in testList1:
		color1 = colorDict1["color"]
		if color1 == findColor1:
			hex1 = colorDict1["hex"]
			print("found color \"{0:s}\" in list of {1:d}, hex is {2:s}".format(color1,len1,hex1))
if (len1:= len(testList1)) != 0:
	for colorDict1 in testList1:
		if (color1 := colorDict1["color"]) == findColor1:
			hex1 = colorDict1["hex"]
			print("found color \"{0:s}\" in list of {1:d}, hex is {2:s}".format(color1,len1,hex1))

Here is the output of the Python code.

found color "green" in list of 3, hex is 00FF00
found color "green" in list of 3, hex is 00FF00