If I have three variables, such as x,y,z=1,2,3, I can use x=y=z but can't use x=(y=z) in python. What's difference between x=y=z and x=(y=z)?
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2 Answers
y=z is an assignment statement, not an expression (as it is in, say, C). It can only be used where a statement is expected. You can't, for instance, do print(y=z). So x=(y=z) is grammatically ill-formed.
x=y=z is a single assignment, not a combination of two assignments. The grammar specifically allows targets to be chained in an assignment statement. The relevant grammar bit:
assignment_stmt ::= (target_list "=")+ (expression_list | yield_expression)
4 Comments
gaussclb
assignment_stmt ::= (target_list "=")+ (expression_list | yield_expression) just has one assignment operator, why you say it can be chained?
user2357112
@gaussclb: The
+ means "one or more" of the thing the + is attached to.gaussclb
Do you mean
assignment_stmt ::= (target_list "=")+ (expression_list | yield_expression | assignment_stmt)?
Sneftel
No, that's not the correct grammar. As userblahblah explained, the
+ means that there can be multiple target_list =s chained in a single expression. target_list = target_list = target_list = expression_list.x=y=z assigns x and y the value stored in z.
In [133]: z = 5
In [134]: x = y = z
In [135]: x
Out[135]: 5
In [136]: y
Out[136]: 5
x=(y=z) wants to assign to x the outcome of the expression in the parenthesis. Unfortunately, the expression y=z is not evaluated this way in Python within the parenthesis.
In [137]: x=(y=z)
File "<ipython-input-137-445a19ecd607>", line 1
x=(y=z)
^
SyntaxError: invalid syntax
If on the otherhand, you were looking to assign the result of the equivalency test of "is y equal to z", then you could do so with the following:
x=(y == z)
In [138]: x = (y == z)
In [139]: x
Out[139]: True
In [140]: y
Out[140]: 5
In [141]: z
Out[141]: 5
