如何将函数定义作为字符串传递给python脚本

Question

I want to pass function definition to a python command line script. What is the best way to do this? I am using python 2. Suppose i have a script like this:

#myscript.py
x = load_some_data()
my_function = load_function_definition_from_command_line()
print my_function(x)

And i want to call it like this: python myscript.py 'def fun(x): return len(x)'

How do i perform the load_function_definition_from_command_line part ?

I imagine a workaround:

1. get the string function definition from command line
2. write it to a file with .py extension in some temp directory
3. load the definition from file using solutions from this question: How to import a module given the full path?
4. execute
5. cleanup

But I am sure there must be a better way.

Answer 1

You can use eval to run code defined in a string. Like so:

import sys

x = load_some_data()
function = eval("".join(sys.argv[1:]))
print(function(x))

With your specific example though you might have to use something like lambda x: len(x)

As @Jan-Spurny rightly points out: "Never, never, never use eval unless you're absolutely sure there is no other way. And even then you should stop and think again."

In my mind the better strategy would be to turn the data loader and executor into a module with a method that takes a function as an argument and runs the desired code. The end result something like this:

import data_loader_and_executor

def function(x):
    return len(x)

data_loader_and_executor.run(function)

Answer 2

Use function exec :

import sys

def load_function_definition_from_command_line():
   exec(sys.argv[1])
   return locals()['fun']

Of course you have to know, how your function will be named, but this can be done by passing to your argument second argument:

 $ python myscript.py 'def fun(x): return len(x)' fun

And then your function will look like:

import sys

def load_function_definition_from_command_line():
   exec(sys.argv[1])
   return locals()[sys.argv[2]]

!!Remember though, that evaluating user input is very dangerous!!

Edit : Since fun would be the only object defined in locals , you can just return first element in locals() :

def load_function_definition_from_command_line():
   exec(sys.argv[1])
   return locals()[0]

Answer 3

You can use eval or exec to create a function in your current namespace.

exec "somefunc(x): return x * 2"
somefunc(2) # 2

Example within your context

python load_function.py "def f(x): return x * 2"

//load_function.py
import sys

exec sys.argv[1]
print f(2)

Command line output:
4

Edit: Obligatory, "It is not wise to execute user input like this."

Answer 4

The most obvious source for the correct answer on how to do this is in the timeit python builtin library.

It is invoked like this:

$ python -m timeit '"-".join(str(n) for n in range(100))'

and you can find the source code here, which uses compile and exec to invoke the code from the command line

https://hg.python.org/cpython/file/2.7/Lib/timeit.py#l143