globals() using For Loop against Generator

[cokofreedom]

if __name__ == '__main__':

print "Globals (For Loop):"
try:
for i in globals():
print "\t%s" % i
except RuntimeError:
print "Only some globals() printed\n"
else:
print "All globals() printed\n"

print "Globals (Generator):"
try:
print "\n".join("\t%s" % i for i in globals())
except RuntimeError:
print "Only some globals() printed\n"
else:
print "All globals() printed\n"

Why is it with a generator I get everything out but with a for loop I
don't? I know that globals is not read-only but I would of expected
the same behaviour from both...

Any thoughts?

 

[Mel]

if __name__ == '__main__':

print "Globals (For Loop):"
try:
for i in globals():
print "\t%s" % i
except RuntimeError:
print "Only some globals() printed\n"
else:
print "All globals() printed\n"

print "Globals (Generator):"
try:
print "\n".join("\t%s" % i for i in globals())
except RuntimeError:
print "Only some globals() printed\n"
else:
print "All globals() printed\n"


Why is it with a generator I get everything out but with a for loop I
don't? I know that globals is not read-only but I would of expected
the same behaviour from both...

Run the for loop in the interpreter, without catching exceptions. You get

__builtins__
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
RuntimeError: dictionary changed size during iteration


Then `print globals()` shows that i has been added to the global
namespace. If you run the for loop a second time, after i exists, the
loop runs fine.

Apparently, generator comprehensions have been optimized so that they
don't expose their working variables. The generator comprehension
won't add i to the global namespace, so all is well.

Mel.