Implement format strings

NEWS.rst

@@ -1,5 +1,13 @@
 .. default-role:: code
 
+Unreleased
+==============================
+
+New Features
+------------------------------
+* Format strings with embedded Hy code (e.g., `f"The sum is {(+ x y)}"`)
+  are now supported, even on Pythons earlier than 3.6.
+
 0.16.0
 ==============================
 

hy/compiler.py

@@ -12,14 +12,15 @@ from funcparserlib.parser import some, many, oneplus, maybe, NoParseError
 from hy.errors import (HyCompileError, HyTypeError, HyLanguageError,
                        HySyntaxError, HyEvalError, HyInternalError)
 
-from hy.lex import mangle, unmangle
+from hy.lex import mangle, unmangle, hy_parse, parse_one_thing, LexException
 
 from hy._compat import (string_types, str_type, bytes_type, long_type, PY3,
-                        PY35, reraise)
+                        PY35, PY36, reraise)
 from hy.macros import require, load_macros, macroexpand, tag_macroexpand
 
 import hy.core
 
+import re
 import pkgutil
 import traceback
 import importlib
@@ -31,6 +32,7 @@ import copy
 import __future__
 
 from collections import defaultdict
+from functools import reduce
 
 if PY3:
     import builtins
@@ -629,8 +631,11 @@ class HyASTCompiler(object):
         elif isinstance(form, HyKeyword):
             body = [HyString(form.name)]
 
-        elif isinstance(form, HyString) and form.brackets is not None:
-            body.extend([HyKeyword("brackets"), form.brackets])
+        elif isinstance(form, HyString):
+            if form.is_format:
+               body.extend([HyKeyword("is_format"), form.is_format])
+            if form.brackets is not None:
+               body.extend([HyKeyword("brackets"), form.brackets])
 
         ret = HyExpression([HySymbol(name)] + body).replace(form)
         return imports, ret, False
@@ -1798,10 +1803,112 @@ class HyASTCompiler(object):
 
     @builds_model(HyString, HyBytes)
     def compile_string(self, string):
+        if type(string) is HyString and string.is_format:
+            # This is a format string (a.k.a. an f-string).
+            return self._format_string(string, str_type(string))
         node = asty.Bytes if PY3 and type(string) is HyBytes else asty.Str
         f = bytes_type if type(string) is HyBytes else str_type
         return node(string, s=f(string))
 
+    def _format_string(self, string, rest, allow_recursion=True):
+        values = []
+        ret = Result()
+
+        while True:
+           # Look for the next replacement field, and get the
+           # plain text before it.
+           match = re.search(r'\{\{?|\}\}?', rest)
+           if match:
+              literal_chars = rest[: match.start()]
+              if match.group() == '}':
+                  raise self._syntax_error(string,
+                      "f-string: single '}' is not allowed")
+              if match.group() in ('{{', '}}'):
+                  # Doubled braces just add a single brace to the text.
+                  literal_chars += match.group()[0]
+              rest = rest[match.end() :]
+           else:
+              literal_chars = rest
+              rest = ""
+           if literal_chars:
+               values.append(asty.Str(string, s = literal_chars))
+           if not rest:
+               break
+           if match.group() != '{':
+               continue
+
+           # Look for the end of the replacement field, allowing
+           # one more level of matched braces, but no deeper, and only
+           # if we can recurse.
+           match = re.match(
+               r'(?: \{ [^{}]* \} | [^{}]+ )* \}'
+                   if allow_recursion
+                   else r'[^{}]* \}',
+               rest, re.VERBOSE)
+           if not match:
+              raise self._syntax_error(string, 'f-string: mismatched braces')
+           item = rest[: match.end() - 1]
+           rest = rest[match.end() :]
+
+           # Parse the first form.
+           try:
+               model, item = parse_one_thing(item)
+           except (ValueError, LexException) as e:
+               raise self._syntax_error(string, "f-string: " + str_type(e))
+
+           # Look for a conversion character.
+           item = item.lstrip()
+           conversion = None
+           if item.startswith('!'):
+               conversion = item[1]
+               item = item[2:].lstrip()
+
+           # Look for a format specifier.
+           format_spec = asty.Str(string, s = "")
+           if item.startswith(':'):
+               if allow_recursion:
+                   ret += self._format_string(string,
+                       item[1:],
+                       allow_recursion=False)
+                   format_spec = ret.force_expr
+               else:
+                   format_spec = asty.Str(string, s=item[1:])
+           elif item:
+               raise self._syntax_error(string,
+                   "f-string: trailing junk in field")
+
+           # Now, having finished compiling any recursively included
+           # forms, we can compile the first form that we parsed.
+           ret += self.compile(model)
+
+           if PY36:
+               values.append(asty.FormattedValue(
+                   string,
+                   conversion = -1 if conversion is None else ord(conversion),
+                   format_spec = format_spec,
+                   value = ret.force_expr))
+           else:
+               # Make an expression like:
+               #    "{!r:{}}".format(value, format_spec)
+               values.append(asty.Call(string,
+                   func = asty.Attribute(
+                       string,
+                       value = asty.Str(string, s =
+                           '{' +
+                           ('!' + conversion if conversion else '') +
+                           ':{}}'),
+                       attr = 'format', ctx = ast.Load()),
+                   args = [ret.force_expr, format_spec],
+                   keywords = [], starargs = None, kwargs = None))
+
+        return ret + (
+           asty.JoinedStr(string, values = values)
+           if PY36
+           else reduce(
+              lambda x, y:
+                  asty.BinOp(string, left = x, op = ast.Add(), right = y),
+              values))
+
     @builds_model(HyList, HySet)
     def compile_list(self, expression):
         elts, ret, _ = self._compile_collect(expression)

hy/lex/lexer.py

@@ -38,7 +38,7 @@ lg.add('HASHOTHER', r'#%s' % identifier)
 # A regexp which matches incomplete strings, used to support
 # multi-line strings in the interpreter
 partial_string = r'''(?x)
-    (?:u|r|ur|ru|b|br|rb)? # prefix
+    (?:u|r|ur|ru|b|br|rb|f|fr|rf)? # prefix
     "  # start string
     (?:
        | [^"\\]             # non-quote or backslash

hy/lex/parser.py

@@ -200,14 +200,22 @@ def t_empty_list(state, p):
 @pg.production("string : STRING")
 @set_boundaries
 def t_string(state, p):
+    s = p[0].value
+    # Detect and remove any "f" prefix.
+    is_format = False
+    if s.startswith('f') or s.startswith('rf'):
+        is_format = True
+        s = s.replace('f', '', 1)
     # Replace the single double quotes with triple double quotes to allow
     # embedded newlines.
     try:
-        s = eval(p[0].value.replace('"', '"""', 1)[:-1] + '"""')
+        s = eval(s.replace('"', '"""', 1)[:-1] + '"""')
     except SyntaxError:
         raise LexException.from_lexer("Can't convert {} to a HyString".format(p[0].value),
                                       state, p[0])
-    return (HyString if isinstance(s, str_type) else HyBytes)(s)
+    return (HyString(s, is_format = is_format)
+        if isinstance(s, str_type)
+        else HyBytes(s))
 
 
 @pg.production("string : PARTIAL_STRING")
@@ -222,7 +230,10 @@ bracket_string_re = next(r.re for r in lexer.rules if r.name == 'BRACKETSTRING')
 def t_bracket_string(state, p):
     m = bracket_string_re.match(p[0].value)
     delim, content = m.groups()
-    return HyString(content, brackets=delim)
+    return HyString(
+        content,
+        is_format = delim == 'f' or delim.startswith('f-'),
+        brackets = delim)
 
 
 @pg.production("identifier : IDENTIFIER")

hy/models.py

@@ -94,8 +94,9 @@ class HyString(HyObject, str_type):
     scripts. It's either a ``str`` or a ``unicode``, depending on the
     Python version.
     """
-    def __new__(cls, s=None, brackets=None):
+    def __new__(cls, s=None, is_format=False, brackets=None):
         value = super(HyString, cls).__new__(cls, s)
+        value.is_format = bool(is_format)
         value.brackets = brackets
         return value
 

tests/compilers/test_ast.py

@@ -10,7 +10,7 @@ from hy.compiler import hy_compile, hy_eval
 from hy.errors import HyCompileError, HyLanguageError, HyError
 from hy.lex import hy_parse
 from hy.lex.exceptions import LexException, PrematureEndOfInput
-from hy._compat import PY3
+from hy._compat import PY3, PY36
 
 import ast
 import pytest
@@ -511,6 +511,18 @@ def test_ast_unicode_vs_bytes():
     assert s('b"\\xa0"') == (bytes([160]) if PY3 else chr(160))
 
 
+@pytest.mark.skipif(not PY36, reason='f-strings require Python 3.6+')
+def test_format_string():
+    assert can_compile('f"hello world"')
+    assert can_compile('f"hello {(+ 1 1)} world"')
+    assert can_compile('f"hello world {(+ 1 1)}"')
+    assert cant_compile('f"hello {(+ 1 1) world"')
+    assert cant_compile('f"hello (+ 1 1)} world"')
+    assert cant_compile('f"hello {(+ 1 1} world"')
+    assert can_compile(r'f"hello {\"n\"} world"')
+    assert can_compile(r'f"hello {\"\\n\"} world"')
+
+
 def test_ast_bracket_string():
     assert s(r'#[[empty delims]]') == 'empty delims'
     assert s(r'#[my delim[fizzle]my delim]') == 'fizzle'

tests/native_tests/language.hy

@@ -1217,6 +1217,72 @@
   (assert (none? (. '"squid" brackets))))
 
 
+(defn test-format-strings []
+  (assert (= f"hello world" "hello world"))
+  (assert (= f"hello {(+ 1 1)} world" "hello 2 world"))
+  (assert (= f"a{ (.upper (+ \"g\" \"k\")) }z" "aGKz"))
+
+  ; Referring to a variable
+  (setv p "xyzzy")
+  (assert (= f"h{p}j" "hxyzzyj"))
+
+  ; Including a statement and setting a variable
+  (assert (= f"a{(do (setv floop 4) (* floop 2))}z" "a8z"))
+  (assert (= floop 4))
+
+  ; Comments
+  (assert (= f"a{(+ 1
+     2 ; This is a comment.
+     3)}z" "a6z"))
+
+  ; Newlines in replacement fields
+  (assert (= f"ey {\"bee
+cee\"} dee" "ey bee\ncee dee"))
+
+  ; Conversion characters and format specifiers
+  (setv p:9 "other")
+  (setv !r "bar")
+  (defn u [s]
+    ; Add a "u" prefix for Python 2.
+    (if PY3
+      s
+      (.replace (.replace s "'" "u'" 1) "  " " " 1)))
+  (assert (= f"a{p !r}" (u "a'xyzzy'")))
+  (assert (= f"a{p :9}" "axyzzy    "))
+  (assert (= f"a{p:9}" "aother"))
+  (assert (= f"a{p !r :9}" (u "a'xyzzy'  ")))
+  (assert (= f"a{p !r:9}" (u "a'xyzzy'  ")))
+  (assert (= f"a{p:9 :9}" "aother    "))
+  (assert (= f"a{!r}" "abar"))
+  (assert (= f"a{!r !r}" (u "a'bar'")))
+
+  ; Fun with `r`
+  (assert (= f"hello {r\"\\n\"}" r"hello \n"))
+  (assert (= f"hello {r\"\n\"}" "hello \n"))
+    ; The `r` applies too late to avoid interpreting a backslash.
+
+  ; Braces escaped via doubling
+  (assert (= f"ab{{cde" "ab{cde"))
+  (assert (= f"ab{{cde}}}}fg{{{{{{" "ab{cde}}fg{{{"))
+  (assert (= f"ab{{{(+ 1 1)}}}" "ab{2}"))
+
+  ; Nested replacement fields
+  (assert (= f"{2 :{(+ 2 2)}}" "   2"))
+  (setv value 12.34  width 10  precision 4)
+  (assert (= f"result: {value :{width}.{precision}}" "result:      12.34"))
+
+  ; Nested replacement fields with ! and :
+  (defclass C [object]
+    (defn __format__ [self format-spec]
+      (+ "C[" format-spec "]")))
+  (assert (= f"{(C) :  {(str (+ 1 1)) !r :x<5}}" "C[  '2'xx]"))
+
+  ; Format bracket strings
+  (assert (= #[f[a{p !r :9}]f] (u "a'xyzzy'  ")))
+  (assert (= #[f-string[result: {value :{width}.{precision}}]f-string]
+    "result:      12.34")))
+
+
 (defn test-import-syntax []
   "NATIVE: test the import syntax."