On 03/01/16 12:27, yehudak . wrote:
> Hi there,
> In a program I wrote the following line (Python 3.5):
>
> print("You've visited", island, '&', new + ".")
>
> A programmer told me that it's a bad habit, and I should have used instead:
>
> print("You've visited {0} {1} {2}{3}".format(island, "&", new, "."))
>
> May I understand why?
There are several reasons although your technique is far from
the worst way of doing things. And the format string here would probably
be better written as:
print("You've visited {0} & {2}.".format(island, new))
ie only put the variables as placeholders.
Why is it better?
1) It is slightly more performant. String addition and
concatenation are relatively slow processes in Python.
Formatting will usually be slightly faster. This is not
a good reason in itself (and in your case with a single
print it's probably irrelevant) but it's one factor. It
does matter more if you are printing inside a loop with
many variables and long strings. (For example assembling
a web page). So it's a good habit to adopt.
2) It improves consistency. You can store the format
string as a variable and then print it out many times
from different parts of your code and it will always
be the same. This makes it much easier to maintain
your program. For example:
fmtString = "You've visited {0} & {2}."
if foo:
print(fmtString.format(foo,bar))
else:
print(fmtString.format(baz,bad))
Now if you want to change the message you only need to
change the text in one place rather than searching your
code to do an edit. Also the user sees exactly the same
formatting, no extra spaces in one message compared
to another for example.
3) formatting provides many extra features to control
justification, leading and spacing. This is especially
important for numeric output where you can define the
number of decimal places, whether a leading sign is
included, padding with zeros etc, etc. You may not
need that initially but if you do have to add it in,
it's trivial with a format string but much more work
if you have lots of hard coded messages all over your
code.
There may be other reasons too but that should give
you some ideas.
--
Alan G
Author of the Learn to Program web site
http://www.alan-g.me.uk/
http://www.amazon.com/author/alan_gauld
Follow my photo-blog on Flickr at:
http://www.flickr.com/photos/alangauldphotos
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