Python: Program that modifies it's self

Question

I am writing a program that is going to run a lot of print output. All I'm really looking to do is have certain pieces of that print output, written directly into my program after it is closes.

Think as if there were a 'save()' function the likeness of the 'print()' that would work like this:

string = "#this my generated comment"
save(string,line)

Line would of course be the line where you want the string saved into the program. Now bare in mind, I don't just want to be limited to adding comments, I would like code that will execute the next time that new line is called.

If the only way to do this is to save the file, edit it and then reload, that is fine. A basic walk-through would be greatly appreciated,

Edit: Alternatively, I am not opposed to the idea of saving the added line into another file and then importing it from the program.

thanks,

I really don't recommend you do this at all. a) It requires that you somehow parse your module. b) It requires specialist knowledge of Python internals c) It's poor practice to store data in source code -- Use a database, JSON/Pickled file, CSV, anything. — James Mills, Dec 11 '13 at 04:35
added to edit: what about saving it into another file and then importing it from within the program so as to add it in? — CodeHard_or_HardCode, Dec 11 '13 at 04:38

score 3 · Answer 1 · answered Dec 11 '13 at 04:38

3

Rather than trying to modify the source code of your running module/program consider any of the following:

Write your data to a JSON/Pickle serialized file.
Write your data to a CSV file.
Use SQLite (comes with most Python distributions)
Use any other kind of real database.

Some helpful links:

answered Dec 11 '13 at 04:38

James Mills

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In this case, the very essence of this program hinges on performing this operation. It may be difficult and perhaps unorthodox but it is necessary. – CodeHard_or_HardCode Dec 11 '13 at 04:48
Perhaps there is some way to append source code? Just being able to add in will solve the problem. – CodeHard_or_HardCode Dec 11 '13 at 04:50
If you are wanting to save "User Data" this is still not a good idea. It is far better to use "some kind of data store". – James Mills Dec 11 '13 at 04:56
its not user data its generated data – CodeHard_or_HardCode Dec 11 '13 at 05:08
Doesn't matter. Store it in a database. – James Mills Dec 11 '13 at 05:10

cronburg · Accepted Answer · 2013-12-11T05:20:05.747

1

The commenters stating that it is bad form / poor practice to modify a python program on the fly are arguably very wrong. This is one of the main features of python which I have personally used on numerous occasions (as a CS grad student working on PL theory). They are however correct that it is bad form to do meta-programming using strings like "x = x + 1". If you want to change the operation of your program on the fly you should use the type function. Also take a look at the following tutorial on meta-programming in python:

http://python-3-patterns-idioms-test.readthedocs.org/en/latest/Metaprogramming.html

If you really need to save an instance of one of your classes across runs of the program, then you would use the pickle library as James Mills pointed out in his answer. It's as simple as:

#!/usr/bin/python

class HelloWorld:

  def __init__(self, string):
    self.string = string

  def run(self):
    print self.string

import pickle
pickle.dump(HelloWorld, open("prog.pkl", 'w'))
HW = pickle.load(open("prog.pkl", 'r'))
x = HW("Hello World")
x.run()

Edit: In order to have "code that will execute the next time that new line is called" you don't want to use inspect.getsource(), instead you should (following the above code):

import types
def newfncn(self):
  print self.string + " 2.0!"
x.run = types.MethodType(newfncn, x)
x.run()

If you really must use strings to edit your code, you can do something like:

import types
def newfncn(self):
  exec("print self.string + \" 2.0!\"")
x.run = types.MethodType(newfncn, x)
x.run()

Which I highly advise against.

edited Dec 11 '13 at 05:20

answered Dec 11 '13 at 04:49

cronburg

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IHMO the op was not talking about **meta programming**. The question was more centered around saving **User Data**. I do agree with you however as I've done a lot of **meta programming** as illustrated in my circuits application framework -- Python has very powerful meta programming capabilities that if used properly are very useful. – James Mills Dec 11 '13 at 04:55
I like where you are going with this, but I don't quite understand. To provide context: I am trying to pull a function back out of inspect.getsource(). Yes I want a stored version of the function but ideally I would like the program to be able to keep running as I described "code that will execute the next time that new line is called". There are other ways to do this yes, but I can't figure how to do this with the inspect.getsource() without modifying the actual program file. – CodeHard_or_HardCode Dec 11 '13 at 05:00
see also (the ladder half of) my preceding question: http://stackoverflow.com/questions/20510020/python-literally-printing-a-function – CodeHard_or_HardCode Dec 11 '13 at 05:01
If you are after a way to dynamically load/unload and run code at runtime, there is a pattern for this called **Plugins**. Python also supports dynamically loading/unloading of modules via the ``__import__()`` function. Otherwise use an appropriate **Plugin** framework/library. Search PyPy for one. – James Mills Dec 11 '13 at 05:12
@NickSimas what kind of changes do you need to make to your code? As James points out if it's numbers / data then you should just save it in a database. If it's making a particular function operate differently then use the types.MethodType() described in my most recent edit. – cronburg Dec 11 '13 at 05:43

Python: Program that modifies it's self

2 Answers2