Multiple assignment and evaluation order in Python

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What is the difference between the following Python expressions:

# First:

x,y = y,x+y

# Second:

x = y
y = x+y

First gives different results than Second.

e.g.,

First:

>>> x = 1
>>> y = 2
>>> x,y = y,x+y
>>> x
2
>>> y
3

Second:

>>> x = 1
>>> y = 2
>>> x = y
>>> y = x+y
>>> x
2
>>> y
4

y is 3 in First and 4 in Second

In an assignment statement, the right-hand side is always evaluated fully before doing the actual setting of variables. So,

x, y = y, x + y

evaluates y (let's call the result ham), evaluates x + y (call that spam), then sets x to ham and y to spam. I.e., it's like

ham = y
spam = x + y
x = ham
y = spam

By contrast,

x = y
y = x + y

sets x to y, then sets y to x (which == y) plus y, so it's equivalent to

x = y
y = y + y

Multiple assignment and evaluation order in Python, Multiple assignment and evaluation order in Python. What is the difference between the following Python expressions: # First: x,y = y,x+y # Second: x = y y = x+y. Multiple assignment and evaluation order in Python. Ask Question Asked 7 years, 11 months ago. Active 1 year, Understanding multiple assignments in Python. 3.

It is explained in the docs in the section entitled "Evaluation order":

... while evaluating an assignment, the right-hand side is evaluated before the left-hand side.

PEP 572 -- Assignment Expressions, to evaluation order; Differences between assignment expressions and assignment Code looks more regular and avoid multiple nested if. Update: also see answers to similar question here: Multiple assignment and evaluation order in Python python Jupyter notebook: No connection to server because websocket connection fails

The first expression:

  1. Creates a temporary tuple with value y,x+y
  2. Assigned in to another temporary tuple
  3. Extract the tuple to variables x and y

The second statement is actually two expressions, without the tuple usage.

The surprise is, the first expression is actually:

temp=x
x=y
y=temp+y

You can learn more about the usage of comma in "Parenthesized forms".

[PDF] Python Evaluation Rules, 6.4 Sequence Slice Access and Assignment Expressions . Note that in order to evaluate one expression, Python evaluates If there are multiple mappings for a particular key, use the last (rightmost) mapping in the. The basic assignment statement does more than assign the result of a single expression to a single variable. The assignment satement also copes nicely with assigning multiple variables at one time. The left and right side must have the same number of elements. For example, the following script has several examples of multiple assignment.

In the second case, you assign x+y to x

In the first case, the second result (x+y) is assigned to y

This is why you obtain different results.

After your edit

This happen because, in the statement

x,y = y,x+y

all variables at the right member are evaluated and, then, are stored in the left members. So first proceed with right member, and second with the left member.

In the second statement

x = y
y = x + y

yo first evaluated y and assign it to x; in that way, the sum of x+y is equivalent to a sum of y+y and not of x+x wich is the first case.

= Assignment, After evaluating this expression all the variables are referencing the same object - integer 10. Another case of assignment is a multi-variable assignment. Order: evaluate a[-1] after = pop(), decreasing the length of a; addition; assignment; The thing is, that a[-1] becomes the value of a[1] (was a[2]) after the pop(), but this happens before the assignment. a[0] = a[0] + a.pop() Works as expected. evaluate a[0] after = pop() addition; assignment

The first one is a tuple-like assignment:

x,y = y,x+y

Where x is the first element of the tuple, and y is the second element, thus what you are doing is:

x = y
y = x+y

Wheras the second is doing a straight assign:

x=y
x=x+y

2. Variables, expressions and statements, If you are not sure what class a value falls into, Python has a function called type which can Triple quoted strings can even span multiple lines: The assignment statement gives a value to a variable: When more than one operator appears in an expression, the order of evaluation depends on the rules of precedence. The Boolean operators and and or are so-called short-circuit operators: their arguments are evaluated from left to right, and evaluation stops as soon as the outcome is determined. For example, if A and C are true but B is false, A and B and C does not evaluate the expression C.

[PDF] Software II: Principles of Programming Languages Why Expressions?, evaluation define the order in which “adjacent” operators The operator associativity rules for expression evaluation All arithmetic, relational, and assignment Python are short-circuit evaluated Perl, Ruby, and Lua allow multiple-target. temp = some_function() x = temp y = temp. Note the order. The leftmost target is assigned first. (A similar expression in C may assign in the opposite order.) From the docs on Python assignment: assigns the single resulting object to each of the target lists, from left to right.

Operators and Expressions in Python – Real Python, Multiple logical operators and operands can be strung together to form compound logical expressions. The xi operands are evaluated in order from left to right. For example, suppose you want to assign a variable s to the value contained  In order to have precisely defined semantics, the proposal requires evaluation order to be well-defined. This is technically not a new requirement, as function calls may already have side effects. Python already has a rule that subexpressions are generally evaluated from left to right.

Python - Multiple Assignment Statement, The assignment satement also copes nicely with assigning multiple variables at one Because of the complete evaluation of the right-hand side, the following  Python supports string and bytes literals and various numeric literals: Evaluation of a literal yields an object of the given type (string, bytes, integer, floating point number, complex number) with the given value. The value may be approximated in the case of floating point and imaginary (complex) literals.

Comments
  • To relate to similar syntax one may find in a codebase x,y = y,x+y is identical to (x,y) = y,x+y, therefore the parenthesis are unnecessary. Does that sound about right?
  • they are not equivalent, the first one is doing x = y, y = x+y, type it into python :)
  • Like the link mentions "All the expressions on the right side are evaluated before any of the assignments". Thank you for the this clarity.