What do numbers starting with 0 mean in python?

Question

When I type small integers with a 0 in front into python, they give weird results. Why is this?

>>> 011
9
>>> 0100
64
>>> 027
23
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Answer 1

The latest Python specification for number literals is in PEP-3127 "Integer Literal Support and Syntax"

The syntax for 011 has been removed in Python 3.0. The syntax for 0o11 has been added in Python 2.6 and it is the only one supported after Python 3.0. There is no __future__ import in Python 2 that would disallow or atleast warn on number literals with leading zeros, so developers have to know what 011 means and that it should be avoided (and that it can actually be avoided).

Answer 2

That is very easy. They are octal numbers.

http://en.wikipedia.org/wiki/Octal

Also there are numbers that are starting with 0x. They are hexadecimal numbers:

>>> 0x51
81

Answer 3

These are numbers represented in base 8 (octal numbers). Some examples:

Python 2 (old format)

Note: these forms only work on Python 2.x.

011 is equal to 1⋅8¹ + 1⋅8⁰ = 9,

0100 is equal to 1⋅8² + 0⋅8¹ + 0⋅8⁰ = 64,

027 is equal to 2⋅8¹ + 7⋅8⁰ = 16 + 7 = 23.

Python 3 (new format)

In Python 3, one must use 0o instead of just 0 to indicate an octal constant, e.g. 0o11 or 0o27, etc. Python 2.x versions >= 2.6 supports both the new and the old format.

0o11 is equal to 1⋅8¹ + 1⋅8⁰ = 9,

0o100 is equal to 1⋅8² + 0⋅8¹ + 0⋅8⁰ = 64,

0o27 is equal to 2⋅8¹ + 7⋅8⁰ = 16 + 7 = 23.

Answer 4

These are octal numbers (base 8, values 0 - 7)

You can convert a decimal number to octal with the oct() function.

In [125]: for i in range(10):
   .....:     print '{:5} {:5}'.format(i, oct(i))
   .....:
    0 0
    1 01
    2 02
    3 03
    4 04
    5 05
    6 06
    7 07
    8 010
    9 011

and convert an octal value to integer with the int() function with the appropriate base (8 in this case):

int(str(17), 8)
Out[129]: 15

The similar set of rules/functions apply for hexadecimal numbers (base 16) using the hex() function.

Answer 5

Numbers in Octal numerical system. Other prefixes are 0x for hexadecimal and 0b for binary.

Answer 6

I have tried it out. I have learned a little bit. From Java I know this as a pitfall: A leading zero introduces an octal number. I have colleagues which didn't resp. don't know that after more than years with experience in Java.

Now I was interesting what the behaviour in Python is. I appreciate that change from Python 2 to Python 3. It is pitfall I have never understood why Java (a young language with focus on "beeing easy") could take over that stupid solution from C.

A leading zero as prefix can be accidentally typed (bad solution). 0x for a hexadecimal number is never accidentally typed (good solution). But 0 and o are to similar therefore I think 0o23 in Python is not a perfect solution. 0c23 (octal) would be a better solution in my opinion.