Integers with leading zeroes

Steven D'Aprano steve at pearwood.info
Wed Jul 22 02:55:48 CEST 2015


On Wed, 22 Jul 2015 03:21 am, Antoon Pardon wrote:

> On 07/19/2015 07:39 AM, Steven D'Aprano wrote:
>> In Python 2, integer literals with leading zeroes are treated as octal,
>> so 09 is a syntax error and 010 is 8.
>> 
>> This is confusing to those not raised on C-style octal literals, so in
>> Python 3 leading zeroes are prohibited in int literals. Octal is instead
>> written using the prefix 0o, similar to hex 0x and binary 0b.
>> 
>> Consequently Python 3 makes both 09 and 010 a syntax error.
>> 
>> However there is one exception: zero itself is allowed any number of
>> leading zeroes, so 00000 is a legal way to write zero as a base-10 int
>> literal.
>> 
>> Does anyone use that (mis)feature?
>> 
> 
> Yes. I like to sometime write numbers with leading zeros.

In Python 2, those numbers will be in octal:

nums = [0000, 0001, 0002, 0003, 
        0004, 0005, 0006, 0007, 
        # 0008 and 0009 are syntax errors
        0010, ... ]


In Python 3, using leading zeroes is always a syntax error, unless all the
numbers are zero:

# Okay
nums = [0000, 0000, 0000, 0000, ... ]

# Fails
nums = [0000, 0001, 0002, 0003, ...]


I'm not interested in the Python 2 case with octal numbers. Can you show an
example of how you would use this in Python 3? Or at least explain what
benefit you get from typing out a block of all zeroes by hand, instead of
(say) this?

nums = [0]*10


> Sometimes these numbers represent codeblocks of a fixed
> number of digits. Always writing those numbers with this
> number of digits helps being aware of this. It is also
> easier for when you need to know how many leading zero's
> such a number has.

I'm not sure what you mean here. Python ints don't have a fixed number of
digits.



-- 
Steven



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