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Elixir-Python Vocabulary

Elixir's syntax essentials for Python tourists

Project goal

Help pythonistas navigate, read and understand Elixir codebases.

• can be used as lookup reference, just like a vocabulary
• each section contains an Elixir syntax feature and its python translation*
• provides links to official documentation

* Warn: translations are closest as possible and therefore not idiomatic

What is NOT

• it's not a comprehensive tour of elixir features
• it's not a way to learn how to write Elixir code
• doesn't explain how things work in Elixir
• doesn't explain why Elixir is designed in such a way

If you are interested in those topics jump to "Further Readings" section

Common syntax

# this is a comment

"hello"  # string

[1, 2, 3]  # list

x = 1  # variable assignement

foo(bar)  # function call

1 + 1 == 2  # comparison operators 

not is_nil(x) and x > 0   # boolean operators 

Atom

Elixir

:hello  

Python

HELLO = "hello"  # ~ global literal constant

Ref: https://hexdocs.pm/elixir/Atom.html

Tuple

Elixir

{1, "b", :c}  

Python

(1, "b", C)  

Ref: https://hexdocs.pm/elixir/Tuple.html

List

Elixir

[1, "b", :c]  # linked list [1 -> ["b" -> [:c]]]

Python

[1, "b", C]  # array list

Ref: https://hexdocs.pm/elixir/List.html

List - Interaction examples

Elixir

x = [1, 2] ++ [3, 4] # (1) concatenate lists

y = [0 | x] # (2) prepend single element

Python

x = [1, 2] + [3, 4] # (1)

y = [0] + x  # (2) 

Ref: https://hexdocs.pm/elixir/List.html

Keyword

Elixir

[{:a, 1}, {:b, 2}]  # list of pairs (atom, value)

[a: 1, b: 2]  # same as above with syntactic sugar

Python

[("a", 1), ("b", 2)]

Ref: https://hexdocs.pm/elixir/Keyword.html

Map

Elixir

%{"a" => 1, "b" => 2} # (1) string keys

%{:a => 1, :b => 2} # (2) atom keys

%{a: 1, b: 2} # (3) same as (2) with syntactic sugar

Python

{"a": 1, "b": 2} # (1), (2) and (3)

Ref: https://hexdocs.pm/elixir/Map.html

Map - Interaction examples

Elixir

x = Map.put(%{a: 1, b: 2}, :c, 3) # (1) add/update an element

x = %{x | c: 3} # (2) update an element (syntactic sugar)

y = Map.take(x, [:a, :b]) # (3) take a subset

Python

x = {**{"a": 1, "b": 2}, "c": 3} # (1) and (2)

y = {k: x[k] for k in x if k in ["a", "b"]} # (3)

Ref: https://hexdocs.pm/elixir/Map.html#summary

If

Elixir

if x >= 0 do 
  :positive
else
  :negative
end

if x >= 0, do: :positive, else: :negative # shorthand

Python

"positive" if x >= 0 else "negative"

Ref: https://hexdocs.pm/elixir/Kernel.html#if/2

Cond

Elixir

cond do # first true condition is entered
  x > 9000 -> "over 9000"
  x > 0 -> "meh"
  _ -> "do you even lift bro" # default branch
end

Python

if x > 9000:
  return "over 9000"
if x > 0: 
  return "meh"
return "do you even lift bro"

Ref: https://hexdocs.pm/elixir/Kernel.SpecialForms.html#cond/1

Pattern matching

Elixir

{a, b, _} = {1, 2, 3}  # (1) a = 1, b = 2, don't care about third elem.

[a, ^b] = [1, 2]  # (2) a = 1 if second element == value of b (pin)

[head | tail] = [1, 2, 3]  # (3) head = 1, tail = [2, 3]

%{a: a} = %{a: 1, b: 2} # (4) a = 1 (~ destructuring)

Python

(a, b, _) = (1, 2, 3) # (1) 

[a, temp] = [1, 2]; if temp != b: raise Exception("Match error!") # (2)

head, tail = lst[0], lst[1:]  # (3)

a = {"a": 1, "b": 2}["a"] # (4)

Ref: getting-started#pattern-matching

Case

Elixir

case result do # first pattern matched is entered
  {:ok, x} when rem(x, 2) -> {:odd, x}
  {:ok, x} -> {:even, x}
  _ -> :error
end

Python

if not isinstance(result, tuple): return "error"
if len(result) != 2: return "error"
if result[0] != "ok": return "error"

x = result[1]
return ("odd", x) if x % 2 else ("even", x)

Ref: https://hexdocs.pm/elixir/Kernel.SpecialForms.html#case/2

With

Elixir

with {:ok, string} <- Map.fetch(map, :key), 
     {value, _} <- Integer.parse(string) do
     value
else # first failed match is sent here
  mismatch -> mismatch 
end

Python

if "key" not in map: return "error"
string = map["key"]
if not string.isnumeric(): return "error"
return int(string) 

Ref: https://hexdocs.pm/elixir/Kernel.SpecialForms.html#with/1

Function

Elixir

def add(x, y) do # (1) named function
  x + y # last expression is the return value
end

def add(x, y), do: x + y # (2) shorthand, same as (1)
defp add(x, y), do: x + y # (3) private function

Python

def add(x, y): # (1) and (2)
  return x + y

def _add(x, y): # (3)
  return x + y

Ref: https://hexdocs.pm/elixir/Kernel.html#def/2

Function - Pattern matching

Elixir

# function definition can be splitted in branches
def op(:add, x, y), do: x + y
def op(:sub, x, y), do: x - y
def op(name, _, _), do: raise "Unknown operation #{name}"

Python

def op(name, x, y):
  if name == "add":
    return x + y
  if name == "sub":
    return x - y
  raise Exception(f"Unknown operation {name}")

Ref: https://hexdocs.pm/elixir/Kernel.html#def/2

Function - Default parameters

Elixir

def increment(x, delta \\ 1), do: x + delta

Python

def increment(x, delta = 1):
  return x + delta

Ref: https://hexdocs.pm/elixir/Kernel.html#def/2

Function - Keyword

Elixir

# named parameters are (often) passed as keywords
String.split("a,b,c", ",", [{:parts, 2}]) # (1)

String.split("a,b,c", ",", parts: 2) # (2) same as (1) syntactic sugar

Python

"a,b,c".split(",", maxsplit=1) # (1) and (2)

Ref: https://hexdocs.pm/elixir/Kernel.html#def/2

Lambda

Elixir

add = fn a, b -> a + b end

add.(1, 2) # note the "."!

Python

add = lambda x, y: x + y

add(1, 2)

Ref: https://hexdocs.pm/elixir/Kernel.SpecialForms.html#fn/1

Lambda - Shorthand

Elixir

add = &(&1 + &2) # (1) reference input parameters by positional index 

add = &Kernel.+/2 # (2) reference an existing function (name / arity)

add_one = &Kernel.+(&1, 1) # (3) ~ partial application

Python

add = lambda x, y: x + y # (1)

import operator
add = operator.add # (2)

add_one = lambda x: operator.add(x, 1) # (3)

Ref: https://hexdocs.pm/elixir/Kernel.SpecialForms.html#&/1

Pipe

Elixir

x         # the previous expression
|> foo()  # is passed as first parameter
|> bar(y) # of the next one

Python

temp = foo(x)
bar(temp, y)

Ref: https://hexdocs.pm/elixir/Kernel.html#%7C%3E/2

Enum

Elixir

Enum.map(lst, &(&1 * &1)) # (1)

Enum.filter(lst, &(&1 > 0)) # (2)

Python

[x * x for x in lst] # (1)

[x for x in lst if x > 0] # (2)

Ref: https://hexdocs.pm/elixir/Enum.html

Enum - Reduce

Elixir

Enum.reduce(lst, 0, fn x, acc -> x + acc end)

Python

acc = 0
for x in lst:
  acc = x + acc

Ref: https://hexdocs.pm/elixir/Enum.html

Module

Elixir

defmodule Foo.Bar do # in file foo/bar.ex
  alias Other.Baz  # allows to refer to Baz without "Other."
  @delta 1         # define constant delta = 1
  def increment(x), do: Baz.f(x) + @delta
end

Python

# in file foo/bar.py
from other import baz
DELTA = 1
def increment(x):
  return baz.f(x) + DELTA

Ref: https://hexdocs.pm/elixir/Kernel.html#defmodule/2

Struct

Elixir

defmodule User do
  defstruct [:name, :age]
end

john = %User{name: "John"} # basically equivalent to a map

Python

class User:
  def __init__(self, name=None, age=None):
    self.name = name
    self.age = age

john = User(name="John")

Ref: https://hexdocs.pm/elixir/Kernel.html#defstruct/1

Typespec

Elixir

@type name :: String.t() | nil

@spec greet(name()) :: String.t() | :error
def greet(nil), do: :error
def greet(name), do: "Hello #{name}"

Python

Name = Optional[str]

def greet(name: Name) -> Union[str, Literal["error"]]:
  return f"Hello {name}" if name is not None else "error"

Ref: https://hexdocs.pm/elixir/typespecs.html

Further readings

• Elixir Getting Started
• Elixir in Action (Book)
• Exercism Elixir Track