exec example - I don't understand

[kepes.krisztian]

Hi !

In


Unifying types and classes in Python 2.2

article I see that:

We can also use the new type in contexts where classic only allows
"real" dictionaries, such as the locals/globals dictionaries for the
exec statement or the built-in function eval():

print a.keys() [1, 2]
exec "x = 3; print x" in a

3

But I dont' understand that:
exec "x = 3; print x" in a

So what this code do ?
Why we need "in a" ?

This get same result !

Thanx for help:
FT

 

[Peter Otten]

But I dont' understand that:
exec "x = 3; print x" in a

So what this code do ?
Why we need "in a" ?

This get same result !

First we make sure there's no variable x in the global namespace:Traceback (most recent call last):
File "<stdin>", line 1, in ?
NameError: name 'x' is not defined

Underneath, the global namespace is just a dictionary with the variable
names as keys and the objects as values.False

Now let's use our own dictionary d as the namespace for the exec statement:
This leaves the global namespace unaffected:False

Instead 1 is stored under the key "x" in the dictionary we provided:

"x" in d True
d["x"]

1

Now let's repeat the same exec statement without explicitly providing a
dictionary. Python will then use globals() as the default. Therefore a
variable x with the value 1 will "magically" appear:1

Peter

 

[Christopher T King]

print a # show the result {1: 3.25, 2: 200}

print a.keys() [1, 2]
exec "x = 3; print x" in a

3

But I dont' understand that:
exec "x = 3; print x" in a

So what this code do ?
Why we need "in a" ?

The 'in a' tells exec to run the code using the dictionary a to read and
store variables. In this case, when x is set equal to 3, it's actually
a['x'] being set to 3. Try these examples to get an idea for what's going
on:

a = {'x': 15}
exec 'print x' in a 15
exec 'print x' NameError: name 'x' is not defined
exec 'x*=3; print x' in a 45
x NameError: name 'x' is not defined
a['x'] 45
exec 'y=10; print y' in a 10
y NameError: name 'y' is not defined
a['y']

10

 

[fowlertrainer]

HI !

Thanx for every answer.
But it is a "bad" thing: when I think to I know something about python I
get some not understanded source code with helpful people's answers, and
them show me that I don't know nothing...

print a # show the result

{1: 3.25, 2: 200}

print a.keys()

[1, 2]

exec "x = 3; print x" in a

3

But I dont' understand that:
exec "x = 3; print x" in a

So what this code do ?
Why we need "in a" ?

The 'in a' tells exec to run the code using the dictionary a to read and
store variables. In this case, when x is set equal to 3, it's actually
a['x'] being set to 3. Try these examples to get an idea for what's going
on:

15

NameError: name 'x' is not defined

45

NameError: name 'x' is not defined

a['x']

45

exec 'y=10; print y' in a

10

y

NameError: name 'y' is not defined

a['y']

10