Re: [Tutor] What is this code doing? What is it?

[eryk sun]

On 5/12/19, Alan Gauld via Tutor  wrote:
>
> They are both very powerful ways of constructing output strings with
> data inserted. {} and format() has a few extra tricks once you get
> into advanced uses, but % style does most of the same things (and
> has the advantage of being used in other languages too so you only
> need to remember one style!).

IMHO, given the choice of learning only one, learn the newer
curly-brace string formatting system. It's extensible since it's based
on a __format__ special method that a type (i.e. class) can override.

Take the Decimal type, for instance. By default it supports 28 digits
of precision. For example:

>>> from decimal import Decimal
>>> n = Decimal('0.12340567891234056789123405669') * 10
>>> n
Decimal('1.234056789123405678912340567')

Decimal implements the __format__ special method with its own
implementation of the "f" format specifier, which faithfully renders
the value as a string.

>>> '{:0.27f}'.format(n)
'1.234056789123405678912340567'

We can also use this directly with the built-in format() function in Python 3:

>>> format(n, '0.27f')
'1.234056789123405678912340567'

On the other hand, if we use a string's percent formatting, its "f"
format specifier has a fixed implementation that first converts a
number to a Python float, as opposed to delegating the string
conversion to the object itself.

>>> float(n)
1.2340567891234058
>>> '%0.27f' % n
'1.234056789123405772912178691'

As you can see in the formatted output, everything after
"1.234056789123405" is wrong. This is because a CPython float is
implemented as a C double-precision value, which only has about 16
decimal digits of precision.
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Re: [Tutor] What is this code doing? What is it?

[Alan Gauld via Tutor]

On 12/05/2019 12:31, Matthew Polack wrote:

> result = str(' Cost: ' + '${:.2f}'.format(cost))
> 
> But I don't understamd what the curly brace part is actually doing:

> ..curly braces apparenly are for dictionaries...but I don't get how this is
> a dictionary..or what this {:} command is actually doing?

Curly braces inside a string are nothing to do with dictionaries or
sets(which also use them).
They are placeholders for inserted data using the format() string
method. Search the docs for string formatting, you will find a page on
the subject with many examples. (or you could read the "Simple
Sequences" topic in my tutorial - about half way down - see below)

There is another, older style of formatting that uses % characters
instead of {}. You will see both styles used regularly in Python
code. Some folks prefer one, some the other.

They are both very powerful ways of constructing output strings with
data inserted. {} and format() has a few extra tricks once you get
into advanced uses, but % style does most of the same things (and
has the advantage of being used in other languages too so you only
need to remember one style!).


-- 
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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