Python types
When calling Python methods, Mojo needs to convert back and forth between native Python objects and native Mojo objects. Most of these conversions happen automatically, but there are a number of cases that Mojo doesn't handle yet. In these cases you may need to do an explicit conversion, or call an extra method.
Mojo types in Python
Mojo primitive types implicitly convert into Python objects. Today we support lists, tuples, integers, floats, booleans, and strings.
For example, given this Python function that prints Python types:
%%python
def type_printer(value):
print(type(value))
(You can ignore the %%python
at the start of the code sample; it's explained
in the note below.)
You can pass this Python function Mojo types with no problem:
type_printer(4)
type_printer(3.14)
type_printer(("Mojo", True))
This is a simplified code example written as a set of Jupyter
notebook cells. The first cell includes the %%python
directive so it's
interpreted as Python. The second cell includes top-level Mojo code. You'd need
to adjust this code to run it elsewhere.
Python types in Mojo
You can also use Python objects from Mojo. For example, Mojo doesn't have a
standard dictionary type yet, but you can work with Python dictionaries in Mojo.
To create a Python dictionary, use the
dict()
method:
from python import Python
fn use_dict() raises:
var dictionary = Python.dict()
dictionary["fruit"] = "apple"
dictionary["starch"] = "potato"
print("Fruit: ", dictionary["fruit"])
Mojo wrapper objects
When you use Python objects in your Mojo code, Mojo adds the
PythonObject
wrapper around
the Python object. This object exposes a number of common double underscore
methods (dunder methods) like __getitem__()
and __getattr__()
, passing them
through to the underlying Python object.
You can explicitly create a wrapped Python object by initializing a
PythonObject
with a Mojo literal:
from python import PythonObject
var py_list: PythonObject = [1, 2, 3, 4]
Most of the time, you can treat the wrapped object just like you'd treat it in
Python. You can use Python's []
operators to access an item in a list, and use
dot-notation to access attributes and call methods. For example:
var n = py_list[2]
py_list.append(5)
If you want to construct a Python type that doesn't have a literal Mojo
equivalent, you can also use the
Python.evaluate()
method. For
example, to create a Python set
:
fn use_py_set() raises:
var py_set = Python.evaluate('set([2, 3, 5, 7, 11])')
var num_items = len(py_set)
print(num_items, " items in set.") # prints "5 items in set"
print(py_set.__contains__(6)) # prints "False"
TODO: You should be able to use the expression 6 in py_set
. However, because
of the way PythonObject
currently works, you need to call the
__contains__()
method directly.
Some Mojo APIs handle PythonObject
just fine, but sometimes you'll need to
explicitly convert a Python value into a native Mojo value.
Currently PythonObject
conforms to the
Intable
and
Stringable
traits, which means you
can convert Python values to Mojo Int
and String
types using the built-in
int()
and
str()
functions, and print Python values
using the built-in print()
function.
PythonObject
also provides the
__bool__()
and
to_float64()
methods for
converting to boolean and floating point values, respectively.
var i: Int = int(py_int)
var s: String = str(py_string)
var b: Bool = py_bool.__bool__()
var f: Float64 = py_float.to_float64()
We mentioned that Python types get wrapped in PythonObject
wrapper.
There is currently one exception to this: Python dictionaries have their own
specialized Mojo wrapper type, Dictionary
. Despite the name, it's not a true
dictionary type, just another kind of wrapper. Most of the time this is
just an implementation detail, but you may notice that the types are different.
Comparing Python types in Mojo
In conditionals, Python objects act like you'd expect them to: Python values
like False
and None
evaluate as false in Mojo, too.
If you need to know the type of the underlying Python object, you can use the
Python.type()
method, which is
equivalent to the Python type()
builtin. You can compare the identity of two
Python objects using the
Python.is_type()
method (which is
equivalent to the Python is
operator):
fn python_types() raises:
from python import Python
from python import PythonObject
var value1: PythonObject = 3.7
var value2 = Python.evaluate("10/3")
var float_type = Python.evaluate("float")
print(Python.type(value1)) # <class 'float'>
print(Python.is_type(Python.type(value1), Python.type(value2))) # True
print(Python.is_type(Python.type(value1), float_type)) # True
print(Python.is_type(Python.type(value1), Python.none())) # False
One TODO item here: The Python.is_type()
method is misleadingly named, since
it doesn't compare types, but object identity.
Further reading
For more information, see Using Mojo with Python on the Modular Blog.