IPython extends Python syntax to allow things like magic commands, and help with the ? syntax. There are several ways to customise how the user’s input is processed into Python code to be executed.
These hooks are mainly for other projects using IPython as the core of their interactive interface. Using them carelessly can easily break IPython!
When the user enters a line of code, it is first processed as a string. By the end of this stage, it must be valid Python syntax.
These transformers act in three groups, stored separately as lists of instances in attributes of IPythonInputSplitter:
An InteractiveShell instance actually has two IPythonInputSplitter instances, as the attributes input_splitter, to tell when a block of input is complete, and input_transformer_manager, to transform complete cells. If you add a transformer, you should make sure that it gets added to both, e.g.:
These transformers may raise SyntaxError if the input code is invalid, but in most cases it is clearer to pass unrecognised code through unmodified and let Python’s own parser decide whether it is valid.
Changed in version 2.0: Added the option to raise SyntaxError.
The simplest kind of transformations work one line at a time. Write a function which takes a line and returns a line, and decorate it with StatelessInputTransformer.wrap():
@StatelessInputTransformer.wrap def my_special_commands(line): if line.startswith("¬"): return "specialcommand(" + repr(line) + ")" return line
The decorator returns a factory function which will produce instances of StatelessInputTransformer using your function.
More advanced transformers can be written as coroutines. The coroutine will be sent each line in turn, followed by None to reset it. It can yield lines, or None if it is accumulating text to yield at a later point. When reset, it should give up any code it has accumulated.
This code in IPython strips a constant amount of leading indentation from each line in a cell:
@CoroutineInputTransformer.wrap def leading_indent(): """Remove leading indentation. If the first line starts with a spaces or tabs, the same whitespace will be removed from each following line until it is reset. """ space_re = re.compile(r'^[ \t]+') line = '' while True: line = (yield line) if line is None: continue m = space_re.match(line) if m: space = m.group(0) while line is not None: if line.startswith(space): line = line[len(space):] line = (yield line) else: # No leading spaces - wait for reset while line is not None: line = (yield line) leading_indent.look_in_string = True
There is an experimental framework that takes care of tokenizing and untokenizing lines of code. Define a function that accepts a list of tokens, and returns an iterable of output tokens, and decorate it with TokenInputTransformer.wrap(). These should only be used in python_line_transforms.
After the code has been parsed as Python syntax, you can use Python’s powerful Abstract Syntax Tree tools to modify it. Subclass ast.NodeTransformer, and add an instance to shell.ast_transformers.
This example wraps integer literals in an Integer class, which is useful for mathematical frameworks that want to handle e.g. 1/3 as a precise fraction:
class IntegerWrapper(ast.NodeTransformer): """Wraps all integers in a call to Integer()""" def visit_Num(self, node): if isinstance(node.n, int): return ast.Call(func=ast.Name(id='Integer', ctx=ast.Load()), args=[node], keywords=) return node