Hacker News Reader

Comment by nneonneo | Hacker News Reader

I mean, who says Python strings are immutable?

  s = "asd"
  import ctypes
  ctypes.memmove(id(s) + ord('1'), b'X', 1)
  print(s) # prints aXd

(Python 3.3+, 64-bit; this is a joke so please do not use this in anything real)

View on HN

Nice!

Can you please explain to a rookie what the ord('1') is for? Is it just a way of writing a magic number of value 49, or is there some significance to it?

It's mostly just a mildly obfuscated way to write 49. It also happens to line up nicely with the index of the character being overwritten (index 1); ord('0') through ord('9') work likewise.

----

I'm personally kind of shocked at how big strings are - the minimum size is 6 native words in size, corresponding to the following fields: refcount, type pointer, string length, string hash (precomputed), flags, and a wstr pointer which just seems to be NULL for most strings. It seems like they could have at least merged the string length and flags together for short strings - a possible subject for a future PEP...

It feels like its just "magic" number. Probably id() is the actual raw pointer to str class and the string starts at offset 48 ?

CPython implementation of the string stores metadata to save memory. The ord('1') simply jumps to the correct memory offset in the string structure. I would think it's easier to remember ord('0') as the offset of the string byte array in the structure than the magic number 48?

Yes, you can can replace that with the magic number 49 if you want, ord('1') == 49 because ord returns the unicode code point for a character.

Documentation is here for the curious: https://docs.python.org/3/library/functions.html#ord

Reply on news.ycombinator.com