Exercise 6.4

Objectives:

Learn to create code with exec()

(a) Experiment with 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.

(b) Creating an `init()` function

In link:ex6_3.txt[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.

(c) Named Tuples

In link:ex2_1.html[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 ...
>>>

[ Solution | Index | Exercise 6.3 | Exercise 6.5 ]

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Exercise 6.4

(a) Experiment with exec()

(b) Creating an __init__() function

(c) Named Tuples

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(b) Creating an `init()` function