Introduction
Objectives:
- Learn to create code with
exec()
Objectives:
exec()
Define a fragment of Python code in a string and try running it:
>>> code = '''
for i in range(n):
print(i, end=' ')
'''
>>> n = 10
>>> exec(code)
0 1 2 3 4 5 6 7 8 9
>>>
That's interesting, but executing random code fragments is not especially useful. A more interesting use of exec()
is in making code such as functions, methods, or classes. Try this example in which we make an __init__()
function for a class.
>>> class Stock:
_fields = ('name', 'shares', 'price')
>>> argstr = ','.join(Stock._fields)
>>> code = f'def __init__(self, {argstr}):\n'
>>> for name in Stock._fields:
code += f' self.{name} = {name}\n'
>>> print(code)
def __init__(self, name,shares,price):
self.name = name
self.shares = shares
self.price = price
>>> locs = { }
>>> exec(code, locs)
>>> Stock.__init__ = locs['__init__']
>>> ## Now try the class
>>> s = Stock('GOOG', 100, 490.1)
>>> s.name
'GOOG'
>>> s.shares
100
>>> s.price
490.1
>>>
In this example, an __init__()
function is made directly from the _fields
variable.
There are no weird hacks involving a special _init()
method or stack frames.
__init__()
functionIn Exercise 6.3, you wrote code that inspected the signature of the __init__()
method to set the attribute names in a _fields
class variable. For example:
class Stock(Structure):
def __init__(self, name, shares, price):
self._init()
Stock.set_fields()
Instead of inspecting the __init__()
method, write a class method create_init(cls)
that creates an __init__()
method from the value of _fields
. Use the exec()
function to do this as shown above. Here's how a user will use it:
class Stock(Structure):
_fields = ('name', 'shares', 'price')
Stock.create_init()
The resulting class should work exactly the name way as before:
>>> s = Stock(name='GOOG', shares=100, price=490.1)
>>> s
Stock('GOOG',100,490.1)
>>> s.shares = 50
>>> s.share = 50
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "structure.py", line 12, in __setattr__
raise AttributeError('No attribute %s' % name)
AttributeError: No attribute share
>>>
Modify the Stock
class in progress to use the create_init()
function as shown.
Verify with your unit tests as before.
While you're at it, get rid of the _init()
and set_fields()
methods on the Structure
class--that approach was kind of weird.
In Exercise 2.1, you experimented with namedtuple
objects in the collections
module. Just to refresh your memory, here is how they worked:
>>> from collections import namedtuple
>>> Stock = namedtuple('Stock', ['name', 'shares', 'price'])
>>> s = Stock('GOOG', 100, 490.1)
>>> s.name
'GOOG'
>>> s.shares
100
>>> s[1]
100
>>>
Under the covers, the namedtuple()
function is creating code as a string and executing it using exec()
. Look at the code and marvel:
>>> import inspect
>>> print(inspect.getsource(namedtuple))
... look at the output ...
>>>
Congratulations! You have completed the Create Code With Exec lab. You can practice more labs in LabEx to improve your skills.