How does EK9's := differ from Python's := walrus operator?

EK9's := and Python's := (walrus) look identical but behave completely differently.

PYTHON := (walrus) is an ASSIGNMENT EXPRESSION:
- Assigns a value AND evaluates to that value
- Can be used INSIDE conditions: if (n := len(data)) > 3:
- Creates the variable on first use
- Returns the assigned value for further use in the expression

EK9 := is an ASSIGNMENT STATEMENT:
- Assigns to an EXISTING variable (must be declared first with <-)
- Cannot be used inside expressions or conditions
- Does not return a value
- Is a standalone statement only

TO ASSIGN AND TEST IN EK9:

Use the <- guard declaration, not :=

  if n <- computeLength()
    use(n)

The guard declares n AND checks if it is set. This is EK9's equivalent of Python's walrus pattern, but uses <- not :=.

QUICK REFERENCE:

  Python:  if (n := len(data)) > 3:       walrus assigns + evaluates
  EK9:     if n <- computeLength() then n > 3    guard declares + checks isSet

  Python:  x := 5  (inside expression)     walrus, creates variable
  EK9:     x <- 5  (standalone)            declaration, creates variable
  EK9:     x := 5  (standalone)            assignment, updates existing variable

Example

defines module qa.gettingstarted.walrusvsassign

  defines function

    computeLength() as pure
      -> items as List of String
      <- rtn as Integer: items.length()

  defines program

    WalrusVsAssignDemo()
      stdout <- Stdout()

      words <- ["hello", "world", "ek9", "language"]
      minimumSize <- 2

      // EK9 guard pattern (replaces Python walrus in conditions)
      if wordCount <- computeLength(words) then wordCount > minimumSize
        stdout.println(`Has ${wordCount} words - enough to process`)

      // := is for updating existing variables (standalone statement only)
      message <- "initial"
      message := "updated"
      stdout.println(message)

      // <- declares, := updates — both are STATEMENTS, never expressions
      counter <- 0
      counter := counter + 1
      stdout.println(`Counter: ${counter}`)

Common mistakes

E01010 — EK9's := is a statement, not an expression. It cannot be used inside conditions. Use the <- guard pattern: 'if v <- expr() then condition'.

Incorrect:

      if (wordCount := computeLength(words)) > minimumSize
        stdout.println(`Has ${$wordCount} words - enough to process`)

Correct:

      if wordCount <- computeLength(words) then wordCount > minimumSize
        stdout.println(`Has ${wordCount} words - enough to process`)

Other ways to ask this

Is EK9's := the same as Python's walrus operator?
Can I use := inside an if condition in EK9?
Why can't I write 'if n := getValue()' in EK9?
How do I assign and test a value in one step in EK9?

Coming from another language?

Python developers moving to EK9: your := walrus patterns become <- guard expressions. EK9's := only works as a standalone statement for updating existing variables.

Keywords: statement, walrus, condition, guard, assign, expression