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Editing: string_interning.md

# String interning

*Interned* strings are conceptually part of an interpreter-global
*set* of interned strings, meaning that:
- no two interned strings have the same content (across an interpreter);
- two interned strings can be safely compared using pointer equality
  (Python `is`).

This is used to optimize dict and attribute lookups, among other things.

Python uses two different mechanisms to intern strings: singletons and
dynamic interning.

## Singletons

The 256 possible one-character latin-1 strings, which can be retrieved with
`_Py_LATIN1_CHR(c)`, are stored in statically allocated arrays,
`_PyRuntime.static_objects.strings.ascii` and
`_PyRuntime.static_objects.strings.latin1`.

Longer singleton strings are marked in C source with `_Py_ID` (if the string
is a valid C identifier fragment) or `_Py_STR` (if it needs a separate
C-compatible name.)
These are also stored in statically allocated arrays.
They are collected from CPython sources using `make regen-global-objects`
(`Tools/build/generate_global_objects.py`), which generates code
for declaration, initialization and finalization.

The empty string is one of the singletons: `_Py_STR(empty)`.

Deep-frozen modules (see `Tools/build/deepfreeze.py`) use either singletons,
or statically allocated strings. These are added to `INTERNED_STRINGS`
at runtime initialization, when deepfreeze modules are loaded.

These sets of singletons (`_Py_LATIN1_CHR`, `_Py_ID`, `_Py_STR`, deepfreeze)
are disjoint.
If you have such a singleton, it (and no other copy) will be interned.

These singletons are interned in a runtime-global lookup table,
`_PyRuntime.cached_objects.interned_strings` (`INTERNED_STRINGS`),
at runtime initialization, and immutable until it's torn down
at runtime finalization.
It is shared across threads and interpreters without any synchronization.

## Dynamically allocated strings

All other strings are allocated dynamically, and have their
`_PyUnicode_STATE(s).statically_allocated` flag set to zero.
When interned, such strings are added to an interpreter-wide dict,
`PyInterpreterState.cached_objects.interned_strings`.

The key and value of each entry in this dict reference the same object.

## Immortality and reference counting

Invariant: Every immortal string is interned.

In practice, this means that you must not use `_Py_SetImmortal` on
a string. (If you know it's already immortal, don't immortalize it;
if you know it's not interned you might be immortalizing a redundant copy;
if it's interned and mortal it needs extra processing in
`_PyUnicode_InternImmortal`.)

The converse is not true: interned strings can be mortal.
For mortal interned strings:
- the 2 references from the interned dict (key & value) are excluded from
  their refcount
- the deallocator (`unicode_dealloc`) removes the string from the interned dict
- at shutdown, when the interned dict is cleared, the references are added back

As with any type, you should only immortalize strings that will live until
interpreter shutdown.
We currently also immortalize strings contained in code objects and similar,
specifically in the compiler and in `marshal`.
These are “close enough” to immortal: even in use cases like hot reloading
or `eval`-ing user input, the number of distinct identifiers and string
constants expected to stay low.

## Internal API

We have the following *internal* API for interning:

- `_PyUnicode_InternMortal`: just intern the string
- `_PyUnicode_InternImmortal`: intern, and immortalize the result
- `_PyUnicode_InternStatic`: intern a static singleton (`_Py_STR`, `_Py_ID`
  or one-byte). Not for general use.

All take an interpreter state, and a pointer to a `PyObject*` which they
modify in place.

The functions take ownership of (“steal”) the reference to their argument,
and update the argument with a *new* reference.
This means:
- They're “reference neutral”.
- They must not be called with a borrowed reference.

## State

The intern state (retrieved by `PyUnicode_CHECK_INTERNED(s)`;
stored in `_PyUnicode_STATE(s).interned`) can be:

- `SSTATE_NOT_INTERNED` (defined as 0, which is useful in a boolean context)
- `SSTATE_INTERNED_MORTAL` (1)
- `SSTATE_INTERNED_IMMORTAL` (2)
- `SSTATE_INTERNED_IMMORTAL_STATIC` (3)

The valid transitions between these states are:

- For dynamically allocated strings:

- 0 -> 1 (`_PyUnicode_InternMortal`)
  - 1 -> 2 or 0 -> 2 (`_PyUnicode_InternImmortal`)

Using `_PyUnicode_InternStatic` on these is an error; the other cases
  don't change the state.

- Singletons are interned (0 -> 3) at runtime init;
  after that all interning functions don't change the state.

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