**Wait for a few seconds here...** It takes a while for Python to load, and the UI doesn'tcurrentlyprovidefeedbackaboutwhenloadingandcompilingiscomplete.(Youcanlookinthedebuggerconsolefor"Python initialization complete"however,ifyou're curious).
## Make a Python cell. Import stuff and use it.
Most of the standard library (at least the parts that make sense) are here and available to use.
%% code {"language":"py"}
# python
import sys
sys.version
%% md
## Basic data types
The basic data types (None, bool, ints, floats, lists, and dicts) are converted from Python to Javascript when they are output and displayed using the standard mechanisms in Iodide.
The Python and Javascript sides can pass objects back and forth.
So, you can set a value in Javascript code:
%% js
// javascript
secret = "Wklv#lv#olnh#pdjlf$"
%% md
...and use it from Python by using `from js import ...`:
%% code {"language":"py"}
# python
from js import secret
decoded = ''.join(chr(ord(x) - 3) for x in secret)
%% md
...and then get it back from Javascript using `pyodide.pyimport`:
%% js
// javascript
var decoded = pyodide.pyimport("decoded")
decoded
%% md
## Custom data types
Non-basic data types, such as class instances, functions, File objects etc., can also be passed between Python and Javascript.
### Using Python objects from Javascript
For example, say we had the following Python function that we wanted to call from Javascript:
%% code {"language":"py"}
# python
def square(x, integer=False):
if integer:
x = int(x)
return x * x
%% md
To call a Python callable from Javascript, we use its `call` method, which takes two arguments: the positional arguments as an array, and the keyword arguments as an object.
%% js
// javascript
var square = pyodide.pyimport("square")
square.call([2.5], {integer: true})
%% md
This is equivalent to the following Python syntax:
%% code {"language":"py"}
# python
square(2.5, integer=True)
%% md
You can also get the attributes of objects in a similar way. Say we had an instance of the following Python custom class:
%% code {"language":"py"}
# python
class Foo:
def __init__(self, val):
self.val = val
foo = Foo(42)
foo
%% md
We can get the value of its `val` property as so:
%% js
// javascript
var foo = pyodide.pyimport("foo")
foo.getattr("val")
%% md
### Using Javascript objects from Python
Likewise, you can use Javascript objects from Python, and since the overloading features of Python are a little more dynamic, the syntax for doing so is also a little bit easier.
%% js
// javascript
function square(x) {
return x*x;
}
%% md
To call this function from Python...
%% code {"language":"py"}
from js import square
square(4)
%% md
## Exceptions
Python exceptions are converted to Javascript exceptions, and they include tracebacks.
%% code {"language":"py"}
x = 5 / 0
%% md
## World DOMination
By using `from js import document`, you can easily access the Web API from Python.
For example, get the title of the document:
%% code {"language":"py"}
# python
from js import document
document.title
%% md
You can set it, too:
%% code {"language":"py"}
# python
document.title = 'Mymindisblown'
%% md
We can set up a special `div` element from a markdown cell, and then manipulate it from Python.
<div id="targetDiv">This is a div we'lltargetfromPython</div>
(Onatechnicallevel,it's important to note that Pyodide doesn'tneedtocopythewholearrayovertotheJavascriptsidetodothis:it's only accessing the parts of the array it needs to make the display.)
