Difference between revisions of "Python assignment operators"

Latest revision as of 13:52, 15 May 2019

Python assignment operators

• Variable Setting (=)

Set a variable by assigning it a value using the *=* sign. Values must be surrounded by double quotes.

VAR = "value"

In this example, *VAR* is set to *value*.

• Early Default Value Assignment (?=)

A variable can be early-assigned a default value using the *?=* assignment operator:

A ?= "1"
B ?= "2"
B ?= "3"

In this example *A* will contain *1* if it was not previously set. *B* will contain *2* because the *?=* assignment operator is immediate meaning that if there are multiple *?=* assignments to a single variable, the first one will be used.

• Late Default Value Assignment (??=)

A variable can be late-assigned a default value using the *??=* assignment operator:

A ??= "1"
B ??= "2"
B ??= "3"

In this example *A* will contain *1* if it was not previously set. *B* will contain *3* because the *??=* assignment operator is a late or lazy assignment operator and assignment will not take place until the end of the parsing process. If there are multiple *??=* assignments to a single variable, the last one will be used.

A ?= following a ??= will set the variable. Only a = will be override the setting of the ?=. Any following ?= and ??= will not change the setting.

• Variable Expansion

Bitbake supports referencing a variables content similar to shell scripting.

A = "jumps over"
B = "The quick brown fox ${A} the lazy dog."
echo $B

This results in the the text *The quick brown fox jumps over the lazy dog.* printed to the console.

• Immediate Variable Expansion (:=)

Typically, Bitbake expands a variable when it is used. The *:=* assignment operator however expands the variable immediately when it is assigned.

A = "11"
B = "B:${A}"
A = "22"
C := "C:${A}"
echo $C $B

This results in *C:22 B:22* printed to the console since the content of * A* is expanded immediately on assignment to variable *C*. Variable *B* did not get expanded until it was actually used in the *echo*statement.

• Appending (+=) and Prepending (=+)

The *+=* and *=+* append and prepend variables respectively while adding a single space between the values.

A = "12"
A += "34"
B = "89"
B =+ "67"

This results in variable *A* containing *12 34* and variable *B* containing *67 89*.

• Appending (.=) and Prepending (=.) without Spaces

The *.=* and *=.* work like the above appending and prepending operators however without placing an additional space between the values.

A = "12"
A .= "34"
B = "89"
B =. "67"

This results in variable *A* containing *1234* and variable *B* containing *6789*.