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*= operator

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  *= operator Cai Gengyang <gengyangcai@gmail.com> - 2015-11-21 05:20 -0800
    Re: *= operator BartC <bc@freeuk.com> - 2015-11-21 13:37 +0000
    Re: *= operator Steven D'Aprano <steve@pearwood.info> - 2015-11-22 00:39 +1100
      Re: *= operator Joel Goldstick <joel.goldstick@gmail.com> - 2015-11-21 08:58 -0500
    Re: *= operator "Frank Millman" <frank@chagford.com> - 2015-11-21 15:46 +0200
      Re: *= operator Cai Gengyang <gengyangcai@gmail.com> - 2015-11-21 05:55 -0800

#99205 — *= operator

This is a piece of code that calculates tax and tip :

def tax(bill):
    """Adds 8% tax to a restaurant bill."""
    bill *= 1.08
    print "With tax: %f" % bill
    return bill

def tip(bill):
    """Adds 15% tip to a restaurant bill."""
    bill *= 1.15
    print "With tip: %f" % bill
    return bill
    
meal_cost = 100
meal_with_tax = tax(meal_cost)
meal_with_tip = tip(meal_with_tax)

Does bill *= 1.08 mean bill = bill * 1.15 ?

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#99206

On 21/11/2015 13:20, Cai Gengyang wrote:
> This is a piece of code that calculates tax and tip :
>
> def tax(bill):
>      """Adds 8% tax to a restaurant bill."""
>      bill *= 1.08
>      print "With tax: %f" % bill
>      return bill
>
> def tip(bill):
>      """Adds 15% tip to a restaurant bill."""
>      bill *= 1.15
>      print "With tip: %f" % bill
>      return bill
>
> meal_cost = 100
> meal_with_tax = tax(meal_cost)
> meal_with_tip = tip(meal_with_tax)
>
> Does bill *= 1.08 mean bill = bill * 1.15 ?

bill *= 1.08 would mean bill = bill*1.08 (not 1.15; I assume that's a typo).

Although you can never be 100% sure in Python (as * or *= could have 
been redefined to do something else), there's no reason to suspect that 
here.

Also, there's probably some technical difference that someone could up 
with (*= doing in-place modification for example), but that shouldn't 
matter when bill is just a number.

-- 
Bartc

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#99207

On Sun, 22 Nov 2015 12:20 am, Cai Gengyang wrote:

> Does bill *= 1.08 mean bill = bill * 1.15 ?

No. It means `bill = bill * 1.08`, not 1.15.



-- 
Steven

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#99210

On Sat, Nov 21, 2015 at 8:39 AM, Steven D'Aprano <steve@pearwood.info>
wrote:

> On Sun, 22 Nov 2015 12:20 am, Cai Gengyang wrote:
>
> > Does bill *= 1.08 mean bill = bill * 1.15 ?
>
> No. It means `bill = bill * 1.08`, not 1.15.
>
>
>
> --
> Steven
>
> --
> https://mail.python.org/mailman/listinfo/python-list
>

Others have answered, but don't forget to try things like this yourself in
the interactive python shell of your choice:

Python 2.7.6 (default, Jun 22 2015, 17:58:13)
[GCC 4.8.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> bill = 1
>>> bill *=1.08
>>> bill
1.08
>>>



-- 
Joel Goldstick
http://joelgoldstick.com/stats/birthdays

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#99208

"Cai Gengyang"  wrote in message 
news:a76b1b5b-4321-41bb-aeca-0dac787752d9@googlegroups.com...

> This is a piece of code that calculates tax and tip :
>
> def tax(bill):
>     """Adds 8% tax to a restaurant bill."""
>     bill *= 1.08
>     print "With tax: %f" % bill
>     return bill
>
> def tip(bill):
>     """Adds 15% tip to a restaurant bill."""
>     bill *= 1.15
>     print "With tip: %f" % bill
>     return bill
>
> meal_cost = 100
> meal_with_tax = tax(meal_cost)
> meal_with_tip = tip(meal_with_tax)
>
> Does bill *= 1.08 mean bill = bill * 1.15 ?

Firstly, I assume that you actually meant 'bill = bill * 1.08' at the end of 
the last line.

Secondly, how can I help you to answer this kind of question yourself.

Here are two ways.

1. Try it out at the interpreter -

c:\>
Python 3.4.3 (v3.4.3:9b73f1c3e601, Feb 24 2015, 22:44:40) [MSC v.1600 64 bit 
(AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> bill = 100
>>> bill *= 1.08
>>> bill

I deliberately omitted the last line. Try it yourself and see what you get.

2. Read the fine manual.

The Index has a section headed 'Symbols'. From there you will find '*=', 
with a link to 'augmented assignment'.

If you follow the link, you will find a detailed explanation. Here is an 
excerpt -

"An augmented assignment expression like x += 1 can be rewritten as x = x + 
1 to achieve a similar, but not exactly equal effect. In the augmented 
version, x is only evaluated once. Also, when possible, the actual operation 
is performed in-place, meaning that rather than creating a new object and 
assigning that to the target, the old object is modified instead."

Frank Millman

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#99209

>>> bill = 100
>>> bill *= 1.08
>>> bill
108.0
>>> 




On Saturday, November 21, 2015 at 9:47:29 PM UTC+8, Frank Millman wrote:
> "Cai Gengyang"  wrote in message 
> news:a76b1b5b-4321-41bb-aeca-0dac787752d9@googlegroups.com...
> 
> > This is a piece of code that calculates tax and tip :
> >
> > def tax(bill):
> >     """Adds 8% tax to a restaurant bill."""
> >     bill *= 1.08
> >     print "With tax: %f" % bill
> >     return bill
> >
> > def tip(bill):
> >     """Adds 15% tip to a restaurant bill."""
> >     bill *= 1.15
> >     print "With tip: %f" % bill
> >     return bill
> >
> > meal_cost = 100
> > meal_with_tax = tax(meal_cost)
> > meal_with_tip = tip(meal_with_tax)
> >
> > Does bill *= 1.08 mean bill = bill * 1.15 ?
> 
> Firstly, I assume that you actually meant 'bill = bill * 1.08' at the end of 
> the last line.
> 
> Secondly, how can I help you to answer this kind of question yourself.
> 
> Here are two ways.
> 
> 1. Try it out at the interpreter -
> 
> c:\>
> Python 3.4.3 (v3.4.3:9b73f1c3e601, Feb 24 2015, 22:44:40) [MSC v.1600 64 bit 
> (AMD64)] on win32
> Type "help", "copyright", "credits" or "license" for more information.
> >>> bill = 100
> >>> bill *= 1.08
> >>> bill
> 
> I deliberately omitted the last line. Try it yourself and see what you get.
> 
> 2. Read the fine manual.
> 
> The Index has a section headed 'Symbols'. From there you will find '*=', 
> with a link to 'augmented assignment'.
> 
> If you follow the link, you will find a detailed explanation. Here is an 
> excerpt -
> 
> "An augmented assignment expression like x += 1 can be rewritten as x = x + 
> 1 to achieve a similar, but not exactly equal effect. In the augmented 
> version, x is only evaluated once. Also, when possible, the actual operation 
> is performed in-place, meaning that rather than creating a new object and 
> assigning that to the target, the old object is modified instead."
> 
> Frank Millman

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