Difference between revisions of "Python assignment operators"

Revision as of 13:41, 15 May 2019

Python assignment operators

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• Variable Setting (=)*

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

VAR = "value"

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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"

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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"

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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. -

• 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

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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

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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"

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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"

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