Difference between revisions of "Python assignment operators"
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== Python assignment operators == | == Python assignment operators == | ||
| − | |||
| − | *Variable Setting (=) | + | * Variable Setting (=) |
Set a variable by assigning it a value using the *=* sign. Values must | Set a variable by assigning it a value using the *=* sign. Values must | ||
be surrounded by double quotes. | be surrounded by double quotes. | ||
| − | + | VAR = "value" | |
| − | VAR = "value" | ||
| − | |||
In this example, *VAR* is set to *value*. | In this example, *VAR* is set to *value*. | ||
| − | |||
| − | *Early Default Value Assignment (?=) | + | |
| + | ---- | ||
| + | |||
| + | |||
| + | * Early Default Value Assignment (?=) | ||
A variable can be early-assigned a default value using the *?=* assignment | A variable can be early-assigned a default value using the *?=* assignment | ||
operator: | operator: | ||
| − | + | A ?= "1" | |
| − | A ?= "1" | + | B ?= "2" |
| − | B ?= "2" | + | B ?= "3" |
| − | B ?= "3" | ||
| − | |||
In this example *A* will contain *1* if it was not previously set. *B* will | In this example *A* will contain *1* if it was not previously set. *B* will | ||
| Line 30: | Line 28: | ||
that if there are multiple *?=* assignments to a single variable, the | that if there are multiple *?=* assignments to a single variable, the | ||
first one will be used. | first one will be used. | ||
| − | |||
| − | *Late Default Value Assignment (??=) | + | |
| + | ---- | ||
| + | |||
| + | |||
| + | * Late Default Value Assignment (??=) | ||
A variable can be late-assigned a default value using the *??=* assignment | A variable can be late-assigned a default value using the *??=* assignment | ||
operator: | operator: | ||
| − | + | A ??= "1" | |
| − | A ??= "1" | + | B ??= "2" |
| − | B ??= "2" | + | B ??= "3" |
| − | B ??= "3" | ||
| − | |||
In this example *A* will contain *1* if it was not previously set. *B* will | In this example *A* will contain *1* if it was not previously set. *B* will | ||
| Line 48: | Line 47: | ||
parsing process. If there are multiple *??=* assignments to a single | parsing process. If there are multiple *??=* assignments to a single | ||
variable, the last one will be used. | variable, the last one will be used. | ||
| − | |||
| − | *Variable Expansion | + | 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 | Bitbake supports referencing a variables content similar to shell | ||
scripting. | scripting. | ||
| − | + | A = "jumps over" | |
| − | A = "jumps over" | + | B = "The quick brown fox ${A} the lazy dog." |
| − | B = "The quick brown fox ${A} the lazy dog." | + | echo $B |
| − | echo $B | + | |
| − | |||
This results in the the text *The quick brown fox jumps over the lazy | This results in the the text *The quick brown fox jumps over the lazy | ||
dog.* printed to the console. | dog.* printed to the console. | ||
| − | |||
| − | *Immediate Variable Expansion (:=) | + | |
| + | ---- | ||
| + | |||
| + | |||
| + | * Immediate Variable Expansion (:=) | ||
Typically, Bitbake expands a variable when it is used. The *:=* assignment | Typically, Bitbake expands a variable when it is used. The *:=* assignment | ||
operator however expands the variable immediately when it is assigned. | operator however expands the variable immediately when it is assigned. | ||
| − | + | A = "11" | |
| − | A = "11" | + | B = "B:${A}" |
| − | B = "B:${A}" | + | A = "22" |
| − | A = "22" | + | C := "C:${A}" |
| − | C := "C:${A}" | + | echo $C $B |
| − | echo $C $B | ||
| − | |||
This results in *C:22 B:22* printed to the console since the content of * | 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 | A* is expanded immediately on assignment to variable *C*. Variable *B* did | ||
not get expanded until it was actually used in the *echo*statement. | not get expanded until it was actually used in the *echo*statement. | ||
| − | |||
| − | *Appending (+=) and Prepending (=+) | + | |
| + | ---- | ||
| + | |||
| + | |||
| + | * Appending (+=) and Prepending (=+) | ||
The *+=* and *=+* append and prepend variables respectively while adding | The *+=* and *=+* append and prepend variables respectively while adding | ||
a single space between the values. | a single space between the values. | ||
| − | + | A = "12" | |
| − | A = "12" | + | A += "34" |
| − | A += "34" | + | B = "89" |
| − | B = "89" | + | B =+ "67" |
| − | B =+ "67" | ||
| − | |||
This results in variable *A* containing *12 34* and variable *B* | This results in variable *A* containing *12 34* and variable *B* | ||
containing *67 89*. | containing *67 89*. | ||
| − | |||
| − | *Appending (.=) and Prepending (=.) without Spaces | + | |
| + | ---- | ||
| + | |||
| + | |||
| + | * Appending (.=) and Prepending (=.) without Spaces | ||
The *.=* and *=.* work like the above appending and prepending operators | The *.=* and *=.* work like the above appending and prepending operators | ||
however without placing an additional space between the values. | however without placing an additional space between the values. | ||
| − | + | A = "12" | |
| − | A = "12" | + | A .= "34" |
| − | A .= "34" | + | B = "89" |
| − | B = "89" | + | B =. "67" |
| − | B =. "67" | ||
| − | |||
This results in variable *A* containing *1234* and variable *B* | This results in variable *A* containing *1234* and variable *B* | ||
containing *6789*. | containing *6789*. | ||
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*.
