Python to Raku - nutshell

Learning Raku from Python, in a nutshell

This page is an attempt to provide a way to learn Raku for folks coming from a Python background. We discuss the equivalent syntax in Raku for a number of Python constructs and idioms.

Basic syntax§

Hello, world§

Let's start with printing "Hello, world!". The put keyword in Raku is the equivalent of print in Python. Like Python 2, parentheses are optional. A newline is added to the end of the line.

Python 2

print "Hello, world!"

Python 3

print("Hello, world!")

Raku

put "Hello, world!"

There is also the say keyword, which behaves similarly, but will call the gist method of its argument.

Raku

my $hello = "Hello, world!";
say $hello;  # also prints "Hello, world!"
             # same as: put $hello.gist

In Python, ' and " are interchangeable. In Raku, both may be used for quoting, but double quotes (") signify that interpolation should be performed. For instance, variables that start with a $, and expressions contained in curly braces are interpolated.

Raku

my $planet = 'earth';
say "Hello, $planet";                 # OUTPUT: «Hello, earth␤»
say 'Hello, $planet';                 # OUTPUT: «Hello, $planet␤»
say "Hello, planet number { 1 + 2 }"; # OUTPUT: «Hello, planet number 3␤»

Statement separators§

In Python, a newline signifies the end of a statement. There are a few exceptions: A backslash before a newline continues a statement across lines. Also if there is an unmatched opening parentheses, square bracket, or curly brace, the statement continues across lines, until the matching curly braces are closed.

In Raku, a semicolon signifies the end of a statement. The semicolon may be omitted if it is the last statement of a block. The semicolon may also be omitted if there is a closing curly brace followed by a newline.

Python

print 1 + 2 + \
    3 + 4
print ( 1 +
    2 )

Raku

say 1 + 2 +
    3 + 4;
say 1 +
    2;

Blocks§

In Python, indentation is used to indicate a block. Raku uses curly braces.

Python

if 1 == 2:
    print("Wait, what?")
else:
    print("1 is not 2.")

Raku

if 1 == 2 {
    say "Wait, what?"
} else {
    say "1 is not 2."
}

Parentheses are optional in both languages for expressions in conditionals, as shown above.

Variables§

In Python, variables are declared and initialized at the same time:

foo = 12
bar = 19

In Raku, the my declarator declares a lexical variable. A variable can be initialized with =. This variable can either be declared first and later initialized or declared and initialized at once.

my $foo;       # declare
$foo = 12;     # initialize
my $bar = 19;  # both at once

Also, as you may have noticed, variables in Raku usually start with sigils -- symbols indicating the type of their container. Variables starting with a $ hold scalars. Variables starting with an @ hold arrays, and variables starting with a % hold a hash (dict). Sigilless variables, declared with a \ but used without them, are bound to the value they are assigned to and are thus immutable.

Please note that, from now on, we are going to use sigilless variables in most examples just to illustrate the similarity with Python. That is technically correct, but in general we are going to use sigilless variables in places where their immutability (or independence of type, when they are used in signatures) is needed or needs to be highlighted.

Python

s = 10
l = [1, 2, 3]
d = { 'a' : 12, 'b' : 99 }

print(s)
print(l[2])
print(d['a'])
# 10, 3, 12

Raku

my $s = 10;
my @l = 1, 2, 3;
my %d = a => 12, b => 99;
my \x = 99;

say $s;
say @l[1];
say %d<a>;  # or %d{'a'}
say x;
# 10, 2, 12, 99

Scope§

In Python, functions and classes create a new scope, but no other block constructor (e.g. loops, conditionals) creates a scope. In Python 2, list comprehensions do not create a new scope, but in Python 3, they do.

In Raku, every block creates a lexical scope.

Python

if True:
    x = 10
print(x)
# x is now 10

Raku

if True {
    my $x = 10
}
say $x
# error, $x is not declared in this scope

my $x;
if True {
    $x = 10
}
say $x
# ok, $x is 10

Python

x = 10
for x in 1, 2, 3:
   pass
print(x)
# x is 3

Raku

my \x = 10;
for 1, 2, 3 -> \x {
    # do nothing
}
say x;
# x is 10

Lambdas in Python can be written as blocks or pointy blocks in Raku.

Python

l = lambda i: i + 12

Raku

my $l = -> $i { $i + 12 }

Another Raku idiom for constructing lambdas is the Whatever star, *.

Raku

my $l = * + 12    # same as above

A * in an expression will become a placeholder for the argument, and transform the expression into a lambda at compile time. Each * in an expression is a separate positional parameter.

See the section below for more constructs regarding subroutines and blocks.

Another example (from the Python FAQ):

Python

squares = []
for x in range(5):
    squares.append(lambda: x ** 2)
print(squares[2]())
print(squares[4]())
# both 16 since there is only one x

Raku

my \squares = [];
for ^5 -> \x {
    squares.append({ x² });
}
say squares[2]();
say squares[4]();
# 4, 16 since each loop iteration has a lexically scoped x,

Note that ^N is like range(N). Similarly, N..^M works like range(N, M) (a list from N to M - 1). The range N..M is a list from N to M. The ^ before or after the .. indicates that the beginning or ending endpoint of the list (or both) should be excluded.

Also, x² is a cute way of writing x ** 2 (which also works fine); the unicode superscript 2 squares a number. Many of the other unicode operators work as you would expect (exponents, fractions, π), but every unicode operator or symbol that can be used in Raku has an ASCII equivalent.

Control flow§

Python has for loops and while loops:

for i in 1, 2:
    print(i)
j = 1
while j < 3:
    print(j)
    j += 1

# 1, 2, 1, 2

Raku also has for loops and while loops:

for 1, 2 -> $i {
    say $i
}
my $j = 1;
while $j < 3 {
    say $j;
    $j += 1
}

(Raku also has a few more looping constructs: repeat...until, repeat...while, until, and loop.)

last leaves a loop in Raku, and is analogous to break in Python. continue in Python is next in Raku.

Python

for i in range(10):
    if i == 3:
        continue
    if i == 5:
        break
    print(i)

Raku

for ^10 -> $i {
    next if $i == 3;
    last if $i == 5;
    say $i;
}

Using if as a statement modifier (as above) is acceptable in Raku, even outside of a list comprehension.

The yield statement within a for loop in Python, which produces a generator, is like a gather/take construct in Raku. These both print 1, 2, 3.

Python

def count():
    for i in 1, 2, 3:
        yield i

for c in count():
    print(c)

Raku

sub count {
    gather {
        for 1, 2, 3 -> $i {
            take $i
        }
    }
}

for count() -> $c {
    say $c;
}

Uses of Python's enumerate() and .items() mechanisms for iterating lists or dict/maps can both be achieved using the same kv method in Raku (because the "key" of a list is its array-like numeric index):

Python

elems = ["neutronium", "hydrogen", "helium", "lithium"]
for i, e in enumerate(elems):
    print("Elem no. %d is %s" % (i, e))
symbols = ["n", "H", "He", "Li"]
elem4Symbol = {s: e for s, e in zip(symbols, elems)}
for symbol, elem in elem4Symbol.items():
    print("Symbol '%s' stands for %s" % (symbol, elem))
# Elem no. 0 is neutronium
# Elem no. 1 is hydrogen
# Elem no. 2 is helium
# Elem no. 3 is lithium
# Symbol 'H' stands for hydrogen
# Symbol 'He' stands for helium
# Symbol 'Li' stands for lithium
# Symbol 'n' stands for neutronium

Raku

my @elems = <neutronium hydrogen helium lithium>;
for @elems.kv -> $i, $e {
    say "Elem no. $i is $e"
}

my @symbols = <n H He Li>;
my %elem-for-symbol;
%elem-for-symbol{@symbols} = @elems;

# Note that the iteration order will differ from Python
for %elem-for-symbol.kv -> $symbol, $element {
    say "Symbol '$symbol' stands for $element";
}

Lambdas, functions and subroutines>§

Declaring a function (subroutine) with def in Python is accomplished with sub in Raku.

def add(a, b):
    return a + b

sub add(\a, \b) {
    return a + b
}

The return is optional; the value of the last expression is used as the return value:

sub add(\a, \b) {
    a + b
}

# using variables with sigils
sub add($a, $b) {
    $a + $b
}

Python 2 functions can be called with positional arguments or keyword arguments. These are determined by the caller. In Python 3, some arguments may be "keyword only". In Raku, positional and named arguments are determined by the signature of the routine.

Python

def speak(word, times):
    for i in range(times):
        print word
speak('hi', 2)
speak(word='hi', times=2)

Raku

Positional parameters:

sub speak($word, $times) {
  say $word for ^$times
}
speak('hi', 2);

Named parameters start with a colon:

sub speak(:$word, :$times) {
  say $word for ^$times
}
speak(word => 'hi', times => 2);
speak(:word<hi>, :times<2>);      # Alternative, more idiomatic

Raku supports multiple dispatch, so several signatures could be made available by declaring a routine as a multi.

multi speak($word, $times) {
  say $word for ^$times
}
multi speak(:$word, :$times) {
    speak($word, $times);
}
speak('hi', 2);
speak(:word<hi>, :times<2>);

Named parameters can be sent using a variety of formats:

sub hello {...};
# all the same
hello(name => 'world'); # fat arrow syntax
hello(:name('world'));  # pair constructor
hello :name<world>;     # <> quotes words and makes a list
my $name = 'world';
hello(:$name);          # lexical var with the same name

Creating an anonymous function can be done with sub, with a block or with a pointy block.

Python

square = lambda x: x ** 2

Raku

my $square = sub ($x) { $x ** 2 };  # anonymous sub
my $square = -> $x { $x ** 2 };     # pointy block
my $square = { $^x ** 2 };          # placeholder variable
my $square = { $_ ** 2 };           # topic variable

Placeholder variables are lexicographically ordered to form positional parameters. Thus these are the same:

my $power = { $^x ** $^y };
my $power = -> $x, $y { $x ** $y };

List comprehensions§

Postfix statement modifiers and blocks can be combined to easily create list comprehensions in Raku.

Python

print([ i * 2 for i in [3, 9]])                      # OUTPUT: «[6, 18]␤»

Raku

say ( $_ * 2 for 3, 9 );                           # OUTPUT: «(6 18)␤»
say ( { $^i * 2 } for 3, 9 );                      # OUTPUT: «(6 18)␤»
say ( -> \i { i * 2 } for 3, 9 );                  # OUTPUT: «(6 18)␤»

Conditionals can be applied, but the if keyword comes first, unlike in Python where the if comes second.

print [ x * 2 for x in [1, 2, 3] if x > 1 ]          # OUTPUT: «[4, 6]␤»

vs

say ( $_ * 2 if $_ > 1 for 1, 2, 3 );              # OUTPUT: «(4 6)␤»

For nested loops, the cross product operator X will help:

print([ i + j for i in [3,9] for j in [2,10] ])       # OUTPUT: «[5, 13, 11, 19]␤»

becomes either of these:

say ( { $_[0] + $_[1] } for (3,9) X (2,10) );      # OUTPUT: «(5 13 11 19)␤»
say ( -> (\i, \j) { i + j } for (3,9) X (2,10) );  # OUTPUT: «(5 13 11 19)␤»
say ( -> ($i, $j) { $i + $j } for (3,9) X (2,10) );# OUTPUT: «(5 13 11 19)␤»
say ( { $^a[0] + $^a[1] } for (3,9) X (2,10) );    # OUTPUT: «(5 13 11 19)␤»

Using map (which is just like Python's map) and grep (which is like Python's filter) is an alternative.

Classes and objects§

Here's an example from the Python docs. First let's go over "instance variables" which are known as attributes in Raku:

Python:

class Dog:
    def __init__(self, name):
        self.name = name

Raku:

class Dog {
    has $.name;
}

For each created class, Raku provides the constructor method new by default which takes named arguments.

Python:

d = Dog('Fido')
e = Dog('Buddy')
print(d.name)
print(e.name)

Raku

my $d = Dog.new(:name<Fido>); # or: Dog.new(name => 'Fido')
my $e = Dog.new(:name<Buddy>);
say $d.name;
say $e.name;

Class attributes in Raku can be declared in a few ways. One way is to just declare a lexical variable and a method for accessing it.

Python:

class Dog:
    kind = 'canine'                # class attribute
    def __init__(self, name):
        self.name = name           # instance attribute
d = Dog('Fido')
e = Dog('Buddy')
print(d.kind)
print(e.kind)
print(d.name)
print(e.name)

Raku:

class Dog {
    my $kind = 'canine';           # class attribute
    method kind { $kind }
    has $.name;                    # instance attribute
}

my $d = Dog.new(:name<Fido>);
my $e = Dog.new(:name<Buddy>);
say $d.kind;
say $e.kind;
say $d.name;
say $e.name;

In order to mutate attributes in Raku, you must use the is rw trait on the attributes:

Python:

class Dog:
    def __init__(self, name):
        self.name = name
d = Dog()
d.name = 'rover'

Raku:

class Dog {
    has $.name is rw;
}
my $d = Dog.new;
$d.name = 'rover';

Inheritance is done using is:

Python

class Animal:
    def jump(self):
        print ("I am jumping")

class Dog(Animal):
    pass

d = Dog()
d.jump()

Raku

class Animal {
    method jump {
        say "I am jumping"
    }
}

class Dog is Animal {
}

my $d = Dog.new;
$d.jump;

Multiple inheritance is possible by using the is trait as many times as required. Alternatively, it can be used in conjunction with the also keyword.

Python

class Dog(Animal, Friend, Pet):
    pass

Raku

class Animal {}; class Friend {}; class Pet {};
...;
class Dog is Animal is Friend is Pet {};

or

class Animal {}; class Friend {}; class Pet {};
...;
class Dog is Animal {
    also is Friend;
    also is Pet;
    ...
}

Decorators§

Decorators in Python are a way of wrapping a function in another one. In Raku, this is done with wrap.

Python

def greeter(f):
    def new():
        print('hello')
        f()
    return new

@greeter
def world():
    print('world')

world();

Raku

sub world {
    say 'world'
}

&world.wrap(sub () {
    say 'hello';
    callsame;
});

world;

An alternative would be to use a trait:

# declare the trait 'greeter'
multi trait_mod:<is>(Routine $r, :$greeter) {
    $r.wrap(sub {
        say 'hello';
        callsame;
    })
}

sub world is greeter {
    say 'world';
}

world;

Context managers§

Context managers in Python declare actions that happen when entering or exiting a scope.

Here's a Python context manager that prints the strings 'hello', 'world', and 'bye'.

class hello:
    def __exit__(self, type, value, traceback):
        print('bye')
    def __enter__(self):
        print('hello')

with hello():
    print('world')

For "enter" and "exit" events, passing a block as an argument would be one option:

sub hello(Block $b) {
    say 'hello';
    $b();
    say 'bye';
}

hello {
    say 'world';
}

A related idea is 'Phasers' which may be set up to run on entering or leaving a block.

{
    LEAVE say 'bye';
    ENTER say 'hello';
    say 'world';
}

input§

In Python 3, the input keyword is used to prompt the user. This keyword can be provided with an optional argument which is written to standard output without a trailing newline:

user_input = input("Say hi → ")
print(user_input)

When prompted, you can enter Hi or any other string, which will be stored in the user_input variable. This is similar to prompt in Raku:

my $user_input = prompt("Say hi → ");
say $user_input; # OUTPUT: whatever you entered.

Tuples§

Python tuples are immutable sequences. The sequence elements do not need to be of the same types.

Python

tuple1 = (1, "two", 3, "hat")
tuple2 = (5, 6, "seven")
print(tuple1[1])                                   # OUTPUT: «two␤»
tuple3 = tuple1 + tuple2
print(tuple3)                                      # OUTPUT: «(1, 'two', 3, 'hat', 5, 6, 'seven')␤»

Raku

Raku does not have a builtin Tuple type. You can get the same behavior from Raku using the List type, or from an external module.

my $list1 = (1, "two", 3, "hat");
my $list2 = (5, 6, "seven");
say $list1[1];                                 # OUTPUT: «two␤»
my $list3 = (slip($list1), slip($list2));
my $list4 = (|$list1, |$list2);                # equivalent to previous line
say $list3;                                    # OUTPUT: «(1, two, 3, hat, 5, 6, seven)␤»