Exploring and understanding Python through surprising snippets.
@@ -350,7 +350,7 @@ Makes sense, right? * 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 `!`. CPython implementation of this rule can be found [here](https://github.com/python/cpython/blob/3.6/Objects/codeobject.c#L19) -  +  + 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 compile-time optimization. This optimization doesn't apply to 3.7.x versions of CPython (check this [issue](https://github.com/satwikkansal/wtfpython/issues/100) for more discussion). + A compile unit in an interactive environment like IPython consists of a single statement, whereas it consists of the entire module in case of modules. `a, b = "wtf!", "wtf!"` is single statement, whereas `a = "wtf!"; b = "wtf!"` are two statements in a single line. This explains why the identities are different in `a = "wtf!"; b = "wtf!"`, and also explain why they are same when invoked in `some_file.py` + The abrupt change in the output of the fourth snippet is due to a [peephole optimization](https://en.wikipedia.org/wiki/Peephole_optimization) technique known as Constant folding. This means the expression `'a'*20` is replaced by `'aaaaaaaaaaaaaaaaaaaa'` during compilation to save a few clock cycles during runtime. Constant folding only occurs for strings having a length of less than 21. (Why? Imagine the size of `.pyc` file generated as a result of the expression `'a'*10**10`). [Here's](https://github.com/python/cpython/blob/3.6/Python/peephole.c#L288) the implementation source for the same. @@ -973,11 +973,11 @@ We didn't assign three `"X"`s, did we? When we initialize `row` variable, this visualization explains what happens in the memory - + And when the `board` is initialized by multiplying the `row`, this is what happens inside the memory (each of the elements `board[0]`, `board[1]` and `board[2]` is a reference to the same list referred by `row`) - +![The word 'row', and the individual elements of the array 'board' (board[0], board[1], board[2]) each have individual arrows pointed at a box that is segmented into three groups. Each group contains two quotation marks, with nothing in the middle.](/images/tic-tac-toe/after_board_initialized.png) We can avoid this scenario here by not using `row` variable to generate `board`. (Asked in [this](https://github.com/satwikkansal/wtfpython/issues/68) issue). diff --git a/images/logo.png b/images/logo.png index 014a63a4..cbca94df 100644 Binary files a/images/logo.png and b/images/logo.png differ diff --git a/images/old_logo.png b/images/old_logo.png new file mode 100644 index 00000000..014a63a4 Binary files /dev/null and b/images/old_logo.png differ diff --git a/images/old_tic-tac-toe.png b/images/old_tic-tac-toe.png new file mode 100644 index 00000000..9c25117b Binary files /dev/null and b/images/old_tic-tac-toe.png differ diff --git a/images/string-intern/old_string_intern.png b/images/string-intern/old_string_intern.png new file mode 100644 index 00000000..6511978a Binary files /dev/null and b/images/string-intern/old_string_intern.png differ diff --git a/images/string-intern/string_intern.png b/images/string-intern/string_intern.png index 6511978a..c678cdac 100644 Binary files a/images/string-intern/string_intern.png and b/images/string-intern/string_intern.png differ diff --git a/images/tic-tac-toe.png b/images/tic-tac-toe.png index 9c25117b..4ff939cf 100644 Binary files a/images/tic-tac-toe.png and b/images/tic-tac-toe.png differ diff --git a/images/tic-tac-toe/after_board_initialized.png b/images/tic-tac-toe/after_board_initialized.png index 616747fb..063fdaf0 100644 Binary files a/images/tic-tac-toe/after_board_initialized.png and b/images/tic-tac-toe/after_board_initialized.png differ diff --git a/images/tic-tac-toe/after_row_initialized.png b/images/tic-tac-toe/after_row_initialized.png index 520d7007..b967bfac 100644 Binary files a/images/tic-tac-toe/after_row_initialized.png and b/images/tic-tac-toe/after_row_initialized.png differ diff --git a/images/tic-tac-toe/old_after_board_initialized.png b/images/tic-tac-toe/old_after_board_initialized.png new file mode 100644 index 00000000..616747fb Binary files /dev/null and b/images/tic-tac-toe/old_after_board_initialized.png differ diff --git a/images/tic-tac-toe/old_after_row_initialized.png b/images/tic-tac-toe/old_after_row_initialized.png new file mode 100644 index 00000000..520d7007 Binary files /dev/null and b/images/tic-tac-toe/old_after_row_initialized.png differ