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From: Dan Strohl <D.Strohl@F5.com>
Newsgroups: comp.lang.python
Subject: RE: Need help understanding list structure
Date: Tue, 3 May 2016 16:52:53 +0000
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Xref: csiph.com comp.lang.python:108085

Take a look at the docs for=20
print() https://docs.python.org/3.5/library/functions.html#print=20
str() https://docs.python.org/3.5/library/stdtypes.html#str
repr() https://docs.python.org/3.5/library/functions.html#repr=20

When you do "print(object)", python will run everything through str() and o=
utput it. =20

Str() will try to return a string representation of the object, what actual=
ly comes back will depend on how the objects author defined it.  If object.=
__str__() has been defined, it will use that, if __str__() is not defined, =
it will use object.__repr__().  If object.__repr__() has not been defined, =
it will something that looks like "object_name object at xxxxxxx".

So, as to your specific questions / comments:

> At the risk of coming across as a complete dunder-head, I think my confus=
ion
> has to do with the type of data the library returns in the list. Any kind=
 of text
> or integer list I manually create, doesn't do this.
Actually, they do, but strings and integers have well defined __str__ and _=
_repr__ methods, and behave pretty well.

So... think about what is actually being passed to the print() function in =
each case:

> print(type(myList))
Passing the string object of the results of type(myList)

> print(len(myList))
Passing the integer object returning from len(myList)

> print(myList[0])
Passing the gedcom ELEMENT object found at location 0 in the list

> print(myList[0:29])
Passing a list object created by copying the items from location 0 to locat=
ion 29 in the original list

> print(myList)
Passing the entire list object

> for x in myList:
>     print(x)
Passing the individual gedcom ELEMENT objects from the list.

So, in each of these cases, you are passing different types of objects, and=
 each one (string, integer, list, gedcom)  will behave differently dependin=
g on how it is coded.

> Why does printing a single item print the actual text of the object?
If you are printing a single item (print(myList[0]), you are printing the _=
_str__ or __repr__ for the object stored at that location.

> Why does printing a range print the "representations" of the objects?
If you are printing a range of objects, you are printing the __str__ or __r=
epr__ for the RANGE OBJECT or LIST object, not the object itself, which app=
arently only checks for a __repr__() method in the contained gedcom ELEMENT=
 objects, which is not defined (see below).

> Why does iterating over the list print the actual text of the objects?
When iterating over the list, you are printing the specific items again (ju=
st like when you did print(myList[0])  )=20

> How can I determine what type of data is in the list?
Try print(type(myList[0])), which will give you the type of data in the fir=
st object in the list (though, keep in mind that the object type could be d=
ifferent in each item in the list).


If we look at the gedcom library (assuming you are talking about this one: =
https://github.com/madprime/python-gedcom/blob/master/gedcom/__init__.py) a=
t the end of the file you can see they defined __str__() in the ELEMENT obj=
ect,  but there is no definition for __repr__(), which matches what we surm=
ised above.

If you want to fix it by editing the gedcom library, you could simply add a=
 line at the end like:
__repr__() =3D __str__()


Hope that helps.

Dan Strohl