Decorators influencing sys._getframe(1)

Question

a.py

import d

d.funcme('blah')

d.py

import sys
import Errors
def argcheck(in_=(), out=(type(None),)):
    def _argcheck(function):
        # do something here
        def __argcheck(*args, **kw):
            print '+++++++++ checking types before calling the func'
            # do something here
            res = function(*args, **kw)
            return res
        return __argcheck
    return _argcheck

@argcheck((str))       <-----
def funcme(name):
    try:
        f = sys._getframe(1)
    except ValueError, err:
        raise Errors.UserError(err)     # stack too deep
        filename, lineno = f.f_globals['__name__'], f.f_lineno
    print filename, lineno

OUTPUT without argcheck decorator (comment out the @argcheck((str))):

$ python a.py  
__main__ 3  

OUTPUT with argcheck decorator:

$ python a.py  
+++++++++ checking types before calling the func  
defines 9  

Questions:

1. What's decorator doing so that it's changing the values for _getframe?

2. How can I preserve the information so it captures the original information i.e __main__ 3 and not defines 9?

Answer 1

The problem is that yourfuncme()function is assuming it has been called directly rather indirectly through something else — such as a decorator. This could be fixed by changing its calling sequence and adding an additionaldepthkeyword argument with a default value which will be passed on to _sys._getframe(). With this scaffolding in place, the decorator can then override the default value. The following will print the same thing whether or not the decorator has been applied:

 1 import sys
 2 import Errors
 3 def argcheck(in_=(), out=(type(None),)):
 4     def _argcheck(function):
 5         # do something here
 6         def __argcheck(*args, **kw):
 7             print '+++++++++ checking types before calling the func'
 8             # do something here
 9             res = function(*args, depth=2, **kw)  # override default depth
10             return res
11         return __argcheck
12     return _argcheck
13
14 @argcheck((str))
15 def funcme(name, depth=1):  # added keyword arg with default value
16     try:
17         f = sys._getframe(depth)   # explicitly pass stack depth wanted
18     except ValueError, err:
19         raise Errors.UserError(err)     # stack too deep
20
21     filename, lineno = f.f_globals['__name__'], f.f_lineno
22     print filename, lineno