todo-placeholder 2.0.1

Use a terminal session to lazily write python source code

todo-placeholder

Installation

pip install todo-placeholder

Placeholder

This is a tool intended to speed the process of development in python.

You're writing your code, and you run into a spot where you don't know exactly what needs to go there.

Placeholder lets you fill that spot with a "placeholder" variable.

At runtime, when it comes time that you actually need to compute the value, "placeholder" will open an interactive session that you can play around in to get the value.

(Optional) When you're done with your session, the placeholder in your code is replaced with expression from your terminal session.

THIS TOOL CAN EDIT YOUR SOURCE CODE. KEEP BACKUPS. I hear git is pretty nice.

wtf

yeah

Simple Example

Say we have my_program.py:

import todo

print(todo.set_placeholder('some_string'))

Run it with python my_program.py and you'll enter an interactive session.

TODO: fill variable "some_string"
When you have an expression that works, press ctrl+D to end the session and
replace the placeholder with the last line you typed.
Alternatively, call exit() to abort.

>>>

This is a code.interact session, which means you have all of python available to you. I recommend learning to use help() and dir() to inspect your environment and see what's available to you.

From here, you should fill in the value of some_string

>>> 'hello world!'
'hello world!'
>>> <ctrl+D>
now exiting InteractiveConsole...

my_program.py will continue execution, except the placeholder call to some_string is replaced with the expression 'hello world!'.

Note: the expression, not the value of the expression. You can use set_placeholder('some_string') later and the same expression will be re-evaluated in whatever context it was accessed from.

It's as if you ran a find+replace job with the last expression you wrote in your terminal session.

When the program completes, you can check my_program.py again:

import todo

print('hello world!')

The placeholder has been replaced by the expression you used in your terminal session.

More realistic example

Say I'm writing a larger program that involves downloading something from the web. Naturally, I'll use requests. But personally, I can never remember exactly what variable in the response holds the HTTP response status.

This my_program.py is a toy example, but it would work the same in a larger program:

import todo
import requests

def get_data(url):
    r = requests.get(url)
    # How do I get the response code here? TODO via placeholder
    if todo.set_placeholder('http_code') != 200:
        raise RuntimeError(f'Error: server responded with error code {todo.set_placeholder("http_code")}')
    return r.text

get_data('http://www.example.com/probably_404')

Running the program will drop me into an interactive session, where the local context of get_data is all available.

TODO: fill variable "http_code"
When you have an expression that works, press ctrl+D to end the session and
replace the placeholder with the last line you typed.
Alternatively, call exit() to abort.

>>> r
<Response [404]>
>>> url
'http://www.example.com/probably_404'
>>> # Now I can use help() to figure out how to get the error code out of r
>>> help(r)

>>> # aha, it's r.status_code
>>> r.status_code
404
>>> <ctrl+D>
now exiting InteractiveConsole...
Traceback (most recent call last):
  File "my_program.py", line 12, in <module>
    get_data('http://www.example.com/probably_404')
  File "my_program.py", line 9, in get_data
    raise RuntimeError(f'Error: server responded with error code {r.status_code}')
RuntimeError: Error: server responded with error code 404

And if I check my_program.py again, the expression has been filled in

import todo
import requests


def get_data(url):
    r = requests.get(url)
    if r.status_code != 200:
        raise RuntimeError(f'Error: server responded with error code {r.status_code}')
    return r.text

get_data('http://www.example.com/probably_404')

set_placeholder

set_placeholder is the simplest way of entering a placeholder context.

The minimum useful context is just a bare call to the function:

import todo
todo.set_placeholder()

More interesting is allowing your placeholder-filled expressions to be re-used with the "key" argument:

import todo
x = todo.set_placeholder('foo')
...
print(todo.set_placeholder('foo'))

In this case, you'll be prompted to fill the first instance of foo, but the second will re-use the expression you entered for the first.

Note that if you pass a key like this, it should be a constant string in order for the rewriter to work properly.

Instead of just filling in expressions, you can also use a call to fill in a statement or even multiple statements, with the replace_mode parameter.

import todo
todo.set_placeholder('foo', replace_mode='expression')  # default
todo.set_placeholder('foo', replace_mode='statement')
todo.set_placeholder('foo', replace_mode='multiline')

Filled expressions are scoped to the replace mode, so this example would trigger three terminal sessions to fill in three different instances of foo.

See the docs for set_placeholder for more advanced usage.

Placeholder Objects

For more control over the placeholder sessions, you can use a placeholder object instead of directly calling set_placeholder.

These are:

• todo.Placeholder (alias for ExpressionPlaceholder)
• todo.ExpressionPlaceholder
• todo.StatementPlaceholder
• todo.MultilinePlaceholder

Example:

import todo

print(todo.set_placeholder('some_string'))

Is equivalent to

import todo
placeholder = todo.Placeholder()

print(placeholder.some_string)

A placeholder object is used through its attributes, and is configured when the object is created.

All placeholder objects take the options rewrite_source and allow_propagation. See parameters for more information.

ExpressionPlaceholder

This is the simplest placeholder: it evaluates a single expression and does not modify the local environment beyond the value of that expression.

StatementPlaceholder

todo.StatementPlaceholder allows for a single python statement, which can be used to do things like assign to a variable.

Example:

import todo

placeholder = todo.StatementPlaceholder()

x = 1
print(x)
placeholder.incr_x
print(x)
placeholder.incr_x
print(x)

1

TODO: fill statement "incr_x"
When you have an expression that works, press ctrl+D to end the session and
replace the placeholder with the last line you typed.
Alternatively, call exit() to abort.

>>> x += 1
>>>
now exiting InteractiveConsole...
2
3

import todo

placeholder = todo.StatementPlaceholder()

x = 1
print(x)
x += 1
print(x)
x += 1
print(x)

MultilinePlaceholder

todo.MultilinePlaceholder allows for replacing a placeholder with many lines of input.

To use this, the placeholder should be the only expression on the line where it is accessed, and you should prefix any lines you want to add to the replacement with '!'

Example:

import todo

placeholder = todo.MultilinePlaceholder()

x = 1
y = -1
print(x, y)
placeholder.incr_xy
print(x, y)
placeholder.incr_xy
print(x, y)

1 -1

TODO: fill statements at "incr_xy"
Play around in your session.
When you want to add a statement to the placeholder, prefix it with "!"
e.g.
>>> !x = x + 10

When your placeholder is complete, press ctrl+D to exit the session and
overwrite the placeholder in source code.
Alternatively, call exit() to abort.

>>> !x += 1
>>> !y -= 1
>>>
now exiting InteractiveConsole...
2 -2
3 -3

import todo

placeholder = todo.MultilinePlaceholder()

x = 1
y = -1
print(x, y)
x += 1
y -= 1
print(x, y)
x += 1
y -= 1
print(x, y)

Parameters

To disable rewriting the source code of the original program, you can pass rewrite_source=False.

By default, Placeholder will refuse to edit the source code of any file except the one it was initialized. This is to prevent things like

placeholder = todo.Placeholder()

some_system_library(placeholder)

from editing important system files that it should not be touching.

To enable editing other files, pass allow_propagation=True.

Caveats

Placeholder objects work by scanning your code for an accessor of the form placeholder_var.some_key. If you access it in an unusual way -- say, __getattribute__, it won't be able to find the item it should replace.

set_placeholder scans for a call to that method with some constant key argument, or else an "anonymous" call with no key.

The rewriter can't tell the difference between a "code" accessor and a "non-code" accessor. So if you have something like

print('placeholder.key = {}'.format(placeholder.key))

Both instances of "placeholder.key" will be replaced in that line, even though only one of them is the actual access point.

Because of how placeholder internals work, keys for Placeholder objects cannot start with an underscore '_'.

If your arrow keys aren't working, try import readline.