I'm a Python developer. What are the key differences I need to know for EK9?

← Getting Started · Ref: Q1006

Key differences for Python developers:

DECLARATION: Python 'x = 5' does both. EK9 separates: 'x <- 5' (create), 'x := 5' (update).

RETURN: Python 'return value'. EK9 has no return — declare '<- rtn as Type' in the signature.

NONE/NULL: Python 'if x is not None'. EK9 has no None — use 'if x?' (suffix ?) or guard 'if x <- expr()'.

LIST COMPREHENSION: Python '[x*2 for x in items if x > 0]'. EK9 uses stream pipeline: 'cat items | filter by isPositive | map with doubleIt | collect as List of Integer'.

INDENTATION: Both use indentation. EK9 uses 2 spaces per level.

TYPING: Python is dynamic. EK9 is statically typed with type inference (x <- 5 infers Integer).

BREAK/CONTINUE: Python has them. EK9 does not — use stream pipelines with head (replaces break) and filter (replaces continue).

WALRUS := Python's := is an expression (assigns AND evaluates). EK9's := is a statement (assigns only, cannot use in conditions). Use <- guard for Python's walrus pattern.

Example

defines module qa.gettingstarted.frompython

  defines function

    doubleValue() as pure
      -> number as Integer
      <- rtn as Integer: number * 2

    isPositive() as pure
      -> number as Integer
      <- rtn as Boolean?

      zeroThreshold <- 0
      rtn: number > zeroThreshold

  defines program

    FromPythonDemo()
      stdout <- Stdout()

      // Python: x = 5 → EK9: x <- 5 (declare), x := 10 (update)
      score <- 5
      score := 10
      stdout.println(`Score: ${score}`)

      // Python: return x*2 → EK9: <- rtn as Integer: number * 2
      doubled <- doubleValue(21)
      stdout.println(`Doubled: ${doubled}`)

      // Python: [x*2 for x in items if x > 0] → EK9: stream pipeline
      numbers <- [-3, 5, -1, 8, 2, -4, 7]
      positiveDoubled <- cat numbers
        | filter by isPositive
        | map with doubleValue
        | collect as List of Integer
      stdout.println(`Result: ${positiveDoubled}`)

Common mistakes

E01072 — 'return' does not exist in EK9. Declare the return variable with '<- rtn as Type: expression' in the function signature.

Incorrect:

      return number * 2

Correct:

    <- rtn as Integer: number * 2

Coming from another language?

Python developers: EK9 uses indentation like Python but is statically typed. The biggest adjustment is no return statement and no None.

Keywords: none, differences, walrus, return, python, migration, comprehension