Skip to content
Permalink
Browse files

Simplify explanation

  • Loading branch information...
satwikkansal committed May 29, 2018
1 parent 2188fa3 commit d2507c2ff35dc6eeb83d6662067bf65aebf260e2
Showing with 2 additions and 2 deletions.
  1. +2 −2 README.md
@@ -200,10 +200,10 @@ Makes sense, right?
+ In the snippets above, strings are implicitly interned. The decision of when to implicitly intern a string is implementation dependent. There are some facts that can be used to guess if a string will be interned or not:
* All length 0 and length 1 strings are interned.
* Strings are interned at compile time (`'wtf'` will be interned but `''.join(['w', 't', 'f']` will not be interned)
* Strings that are not composed of ASCII letters, digits or underscores, are not interned. This explains why `'wtf!'` was not interned due to `!`. Implementation can be found [here](https://github.com/python/cpython/blob/3.6/Objects/codeobject.c#L19)
* Strings that are not composed of ASCII letters, digits or underscores, are not interned. This explains why `'wtf!'` was not interned due to `!`. Cpython implementation of this rule can be found [here](https://github.com/python/cpython/blob/3.6/Objects/codeobject.c#L19)
<img src="https://github.com/images/string-intern/string_intern.png" alt="">
+ When `a` and `b` are set to `"wtf!"` in the same line, the Python interpreter creates a new object, then references the second variable at the same time. If you do it on separate lines, it doesn't "know" that there's already `wtf!` as an object (because `"wtf!"` is not implicitly interned as per the facts mentioned above). It's a compiler optimization and specifically applies to the interactive environment.
+ Constant folding is a technique for [peephole optimization](https://en.wikipedia.org/wiki/Peephole_optimization) in Python. This means the expression `'a'*20` is replaced by `'aaaaaaaaaaaaaaaaaaaa'` during compilation to reduce few clock cycles during runtime. This kind of string (multiplying a string and a number) can be interned if it have length not larger than 20, or it'll be discarded by peephole optimization (Why? Imagine the size of `.pyc` file generated as a result of the expression `'a'*10**10`). Implementation can be found [here](http://www.oddjack.com/?certs=python/cpython/blob/3.6/Python/peephole.c#L288)
+ Constant folding is a technique for [peephole optimization](https://en.wikipedia.org/wiki/Peephole_optimization) in Python. This means the expression `'a'*20` is replaced by `'aaaaaaaaaaaaaaaaaaaa'` during compilation to reduce few clock cycles during runtime. Constant folding only occurs for strings having length less than 20. (Why? Imagine the size of `.pyc` file generated as a result of the expression `'a'*10**10`). [Here's](http://www.oddjack.com/?certs=python/cpython/blob/3.6/Python/peephole.c#L288) the implementation source for the same.
---

0 comments on commit d2507c2

Please sign in to comment.
You can’t perform that action at this time.