Hy <-> Python interop¶
“Keep in mind we’re not Clojure. We’re not Common Lisp. We’re Homoiconic Python, with extra bits that make sense.” — Hy Style Guide
Despite being a Lisp, Hy aims to be fully compatible with Python. That means every Python module or package can be imported in Hy code, and vice versa.
Using Python from Hy¶
Using Python from Hy is nice and easy, you just have to import it.
If you have the following in
greetings.py in Python:
def greet(name): print("hello," name)
You can use it in Hy:
(import greetings) (.greet greetings "foo") ; prints "hello, foo"
You can also import
.pyc bytecode files, of course.
A quick note about mangling¶
In Python, snake_case is used by convention. Lisp dialects tend to use dashes instead of underscores, so Hy does some magic to give you more pleasant names.
In the same way,
UPPERCASE_NAMES from Python can be used
You can use either the original names or the new ones.
def function_with_a_long_name(): print(42) FOO = "bar"
Then, in Hy:
(import example) (.function-with-a-long-name example) ; prints "42" (.function_with_a_long_name example) ; also prints "42" (print (. example *foo*)) ; prints "bar" (print (. example FOO)) ; also prints "bar"
Mangling isn’t that simple; there is more to discuss about it, yet it doesn’t belong in this section.
Using Hy from Python¶
Suppose you have written some useful utilities in Hy, and you want to use them in regular Python, or to share them with others as a package. Or suppose you work with somebody else, who doesn’t like Hy (!), and only uses Python.
In any case, you need to know how to use Hy from Python. Fear not, for it is easy.
If you save the following in
(setv *this-will-be-in-caps-and-underscores* "See?") (defn greet [name] (print "hello from hy," name))
Then you can use it directly from Python, by importing Hy before importing the module. In Python:
import hy import greetings greetings.greet("Foo") # prints "Hello from hy, Foo" print(THIS_WILL_BE_IN_CAPS_AND_UNDERSCORES) # prints "See?"
If you create a package with Hy code, and you do the
import hy in
__init__.py, you can then directly include the package. Of course, Hy still
has to be installed.
You can also compile a module with
hyc, which gives you a
.pyc file. You
can import that file. Hy does not really need to be installed ; however, if in
your code, you use any symbol from Hy Core, a corresponding
statement will be generated, and Hy will have to be installed.
Even if you do not use a Hy builtin, but just another function or variable with
the name of a Hy builtin, the
import will be generated. For example, the previous code
causes the import of
Bottom line: in most cases, Hy has to be installed.
Launching a Hy REPL from Python¶
You can use the function
run_repl() to launch the Hy REPL from Python:
>>> import hy.cmdline >>> hy.cmdline.run_repl() hy 0.12.1 using CPython(default) 3.6.0 on Linux => (defn foo  (print "bar")) => (test) bar
If you want to print the Python code Hy generates for you, use the
>>> import hy.cmdline >>> hy.cmdline.run_repl(spy=True) hy 0.12.1 using CPython(default) 3.6.0 on Linux => (defn test  (print "bar")) def test(): return print('bar') => (test) test() bar
Evaluating strings of Hy code from Python¶
Evaluating a string (or
file object) containing a Hy expression requires
two separate steps. First, use the
read_str function (or
read for a
file object) to turn the expression into a Hy model:
>>> import hy >>> expr = hy.read_str("(- (/ (+ 1 3 88) 2) 8)")
Then, use the
eval function to evaluate it:
>>> hy.eval(expr) 38.0