[Python] Using a function defined in an exec'ed string in Python 3 [duplicate]

Why does the following python3 code produces an error?

a='''
def x():
print(42)
'''

class Test:
def __init__(self):
exec(a)
x()

t = Test()


Results in this message:

Traceback (most recent call last):
File "bug.py", line 11, in <module>
t = Test()
File "bug.py", line 9, in __init__
x()
NameError: global name 'x' is not defined

Continue reading...