@PyNoobs: The Fundamental Five Built-in Functions, and Beyond!

rantingrick rantingrick at gmail.com
Thu Jul 28 17:34:34 EDT 2011

On Jul 26, 9:53 pm, Terry Reedy <tjre... at udel.edu> wrote:
> On 7/26/2011 8:01 PM, rantingrick wrote:
> > Most new user think that printing an object to stdout is all they'll
> > ever need. However when you call print -- or sys.stdout.write(object)
> > -- you are only seeing a "friendly" version of the object.
> This mostly applies to strings, which *do* have 2 printed versions.  It
> is occasionally very important for debugging string problems.

Actually you have to be careful of objects too. Consider this:

py> import Tkinter
py> root = Tkinter.Tk()
py> print root
py> print repr(root)
<Tkinter.Tk instance at 0x02BDC4E0>
py> label = Tkinter.Label(root)
py> print label
py> print repr(label)
<Tkinter.Label instance at 0x02BE18F0>

...as you can see, if someone overrides __str__ you can get some very
confusing return values from a naked print(obj) -- which simply calls
"obj.__str__()" under the covers.

> > --------------------------------------------------
> >   5. id()
> > --------------------------------------------------
> > http://docs.python.org/py3k/library/functions.html#id
> >
> > [...}
> >
> This is the most dangerous of the builtins, as it sometimes mislead
> newbies into 'discovering' non-existent 'bugs'. The main point is that
> the id of immutable objects is mostly an irrelevant implementation
> detail, while the id of mutables may be critical. Lists of lists is a
> particular area where id() is really useful.

Yes this one can cause some major confusion to newcomers. I wonder if
python should throw a warning anytime you call id() on an immutable

py> a = "12345"
py> id(a)
py> b = "12345"
py> id(b)

That first call to id should have thrown a warning:

 Traceback (most recent call last):
   File "<pyshell#5>", line 1, in <module>
 Warning: Comparing ids of immutable types is unreliable!

and an Exception if someone tries an assertion!

 py> assert id(a) != id(b)

 Traceback (most recent call last):
   File "<pyshell#6>", line 1, in <module>
     assert id(a) != id(b)
 Exception: Cannot compare immutable types!

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