exec and global puzzle

Steve Holden steve at holdenweb.com
Mon Dec 6 14:51:21 CET 2004


Andr? Roberge wrote:

> I have the following two files:
> 
> #--testexec.py--
> def exec_code(co):
>     try:
>         exec co
>     except:
>         print "error"
> 
> #-- test.py--
> import thread
> import testexec
> import time
> 
> code = "def a():\n print 'a'\n\n" +\
>        "def c():\n a()\n\nc()"
> 
> code2 = "def a():\n print 'a'\n\n" +\
>        "def c():\n global a\n a()\n\nc()"
> 
> print " exec code - no global"
> exec code
> print " exec from thread - no global"
> thread.start_new(testexec.exec_code, (code,))
> time.sleep(1)
> print "\n exec code2 - with global"
> exec code2
> print " exec from thread - with global"
> thread.start_new(testexec.exec_code, (code2,))
> #-----------------------
> 
> Here's the output when I execute test.py:
> 
>  exec code - no global
> a
>  exec from thread - no global
> error
> 
>  exec code2 - with global
> a
>  exec from thread - with global
> a
> #---------
> Without the global statement, I get an error when trying to execute
> the code.
> I don't understand why I need to use the global statement within the
> definition of c() in order for it to know what a() is.  If I define
> exec_code() within test.py and use it there, I do not get any error,
> with or without the use of a global statement.
> 
> Andre

I have taken the liberty of restructuring your program slightly, by 
using a "from" to explicitly import the function you need, and by using 
extended string literals (""" ... """) to make the code easier to read. 
I have also included print statements to show the contents of the local 
and global namespaces after the definition of function a(). After these 
transformations it looks like this:

import thread
from  testexec import exec_code
import time

code = """\
def a():
   print 'a'

def c():
   a()

c()
"""

code2 = """\
def a():
   print 'a'

def c():
  global a
  a()

c()
"""

print " exec code - no global"
exec code
print " exec from thread - no global"
thread.start_new(exec_code, (code,))
time.sleep(1)
print "\n exec code2 - with global"
exec code2
print " exec from thread - with global"
thread.start_new(exec_code, (code2,))
time.sleep(1)

(OK, I added a final sleep so I saw the output from the second thread).

The reason you are seeing this behavior lies in the behavior of the exec 
statement. The full syntax for that statement is

exec_stmt  ::=  "exec" expression ["in" expression ["," expression]]

The second and third expressions are mappings that will be used as 
namespaces. Since you don't provide either, the interpreter uses the 
current scope (whose contents can be determined using the locals() 
function) for both namespaces, so it doesn't matter whether the function 
is added to the local or the global namespace.

When using threads, however (and, by the way, the usual advice is to use 
the threading module, which has a more convenient API), the namespaces 
are clearly different. The a() function is being added to the local 
namespace for the exec.

So ultimately it's to do with namespaces, as many of the more perplexing 
problems in Python are. I hope this helps establish exactly *why* you 
see what you do.

regards
  Steve
-- 
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