How do I execute a string containing Python code in Python?


How do I execute a string containing Python code in Python?

3/31/2009 4:14:25 PM

Accepted Answer

In the example a string is executed as code using the exec function.

import sys
import StringIO

# create file-like string to capture output
codeOut = StringIO.StringIO()
codeErr = StringIO.StringIO()

code = """
def f(x):
    x = x + 1
    return x

print 'This is my output.'

# capture output and errors
sys.stdout = codeOut
sys.stderr = codeErr

exec code

# restore stdout and stderr
sys.stdout = sys.__stdout__
sys.stderr = sys.__stderr__

print f(4)

s = codeErr.getvalue()

print "error:\n%s\n" % s

s = codeOut.getvalue()

print "output:\n%s" % s

3/31/2009 4:14:38 PM

For statements, use exec(string) (Python 2/3) or exec string (Python 2):

>>> mycode = 'print "hello world"'
>>> exec(mycode)
Hello world

When you need the value of an expression, use eval(string):

>>> x = eval("2+2")
>>> x

However, the first step should be to ask yourself if you really need to. Executing code should generally be the position of last resort: It's slow, ugly and dangerous if it can contain user-entered code. You should always look at alternatives first, such as higher order functions, to see if these can better meet your needs.

Licensed under: CC-BY-SA with attribution
Not affiliated with: Stack Overflow