require list comprehensions to start with a for clause

Doc/ref/ref5.tex

@@ -153,7 +153,7 @@ square brackets:
 
 \begin{verbatim}
 list_display:   "[" [listmaker] "]"
-listmaker:   expression ( list_iter | ( "," expression)* [","] )
+listmaker:   expression ( list_for | ( "," expression)* [","] )
 list_iter:   list_for | list_if
 list_for:    "for" expression_list "in" testlist [list_iter]
 list_if:     "if" test [list_iter]
@@ -164,7 +164,8 @@ by providing either a list of expressions or a list comprehension.
 When a comma-separated list of expressions is supplied, its elements are
 evaluated from left to right and placed into the list object in that
 order.  When a list comprehension is supplied, it consists of a
-single expression followed by one or more "for" or "if" clauses.  In this
+single expression followed by at least one "for" clause and zero or  more
+"for" or "if" clauses.  In this 
 case, the elements of the new list are those that would be produced
 by considering each of the "for" or "if" clauses a block, nesting from
 left to right, and evaluating the expression to produce a list element

Doc/tut/tut.tex

@@ -1755,10 +1755,15 @@ item, then to the result and the next item, and so on.  For example,
 
 \subsection{List Comprehensions}
 
-List comprehensions provide a concise way to create lists without
-resorting to use of the \function{map()} or \function{filter()}
-functions.  The resulting construct tends often to be clearer than use
-of those functions.
+List comprehensions provide a concise way to create lists without resorting
+to use of \function{map()}, \function{filter()} and/or \keyword{lambda}.
+The resulting list definition tends often to be clearer than lists built
+using those constructs.  Each list comprehension consists of an expression
+following by a \keyword{for} clause, then zero or more \keyword{for} or
+\keyword{if} clauses.  The result will be a list resulting from evaluating
+the expression in the context of the \keyword{for} and \keyword{if} clauses
+which follow it.  If the expression would evaluate to a tuple, it must be
+parenthesized.
 
 \begin{verbatim}
 >>> freshfruit = ['  banana', '  loganberry ', 'passion fruit  ']
@@ -1771,6 +1776,17 @@ of those functions.
 [12, 18]
 >>> [3*x for x in vec if x < 2]
 []
+>>> [{x: x**2} for x in vec]
+[{2: 4}, {4: 16}, {6: 36}]
+>>> [[x,x**2] for x in vec]
+[[2, 4], [4, 16], [6, 36]]
+>>> [x, x**2 for x in vec]	# error - parens required for tuples
+  File "<stdin>", line 1
+    [x, x**2 for x in vec]
+               ^
+SyntaxError: invalid syntax
+>>> [(x, x**2) for x in vec]
+[(2, 4), (4, 16), (6, 36)]
 >>> vec1 = [2, 4, 6]
 >>> vec2 = [4, 3, -9]
 >>> [x*y for x in vec1 for y in vec2]

Grammar/Grammar

@@ -77,7 +77,7 @@ term: factor (('*'|'/'|'%') factor)*
 factor: ('+'|'-'|'~') factor | power
 power: atom trailer* ('**' factor)*
 atom: '(' [testlist] ')' | '[' [listmaker] ']' | '{' [dictmaker] '}' | '`' testlist '`' | NAME | NUMBER | STRING+
-listmaker: test ( list_iter | (',' test)* [','] )
+listmaker: test ( list_for | (',' test)* [','] )
 lambdef: 'lambda' [varargslist] ':' test
 trailer: '(' [arglist] ')' | '[' subscriptlist ']' | '.' NAME
 subscriptlist: subscript (',' subscript)* [',']

Lib/test/output/test_grammar

@@ -55,4 +55,5 @@ classdef
 [(1, 'Apple'), (1, 'Banana'), (1, 'Coconut'), (2, 'Apple'), (2, 'Banana'), (2, 'Coconut'), (3, 'Apple'), (3, 'Banana'), (3, 'Coconut'), (4, 'Apple'), (4, 'Banana'), (4, 'Coconut'), (5, 'Apple'), (5, 'Banana'), (5, 'Coconut')]
 [(1, 'Banana'), (1, 'Coconut'), (2, 'Banana'), (2, 'Coconut'), (3, 'Banana'), (3, 'Coconut'), (4, 'Banana'), (4, 'Coconut'), (5, 'Banana'), (5, 'Coconut')]
 good: got a SyntaxError as expected
+good: got a SyntaxError as expected
 [('Boeing', 'Airliner'), ('Boeing', 'Engine'), ('Ford', 'Engine'), ('Macdonalds', 'Cheeseburger')]

Lib/test/test_grammar.py

@@ -578,6 +578,12 @@ try:
 except SyntaxError:
     print "good: got a SyntaxError as expected"
 
+try:
+    eval("[x if y]")
+    print "FAIL: should have raised a SyntaxError!"
+except SyntaxError:
+        print "good: got a SyntaxError as expected"
+
 suppliers = [
   (1, "Boeing"),
   (2, "Ford"),

Python/compile.c

@@ -1043,7 +1043,7 @@ com_list_iter(struct compiling *c,
 static void
 com_list_comprehension(struct compiling *c, node *n)
 {
-	/* listmaker: test list_iter */
+	/* listmaker: test list_for */
 	char tmpname[12];
 	sprintf(tmpname, "__%d__", ++c->c_tmpname);
 	com_addoparg(c, BUILD_LIST, 0);
@@ -1052,7 +1052,7 @@ com_list_comprehension(struct compiling *c, node *n)
 	com_addopnamestr(c, LOAD_ATTR, "append");
 	com_addopnamestr(c, STORE_NAME, tmpname);
 	com_pop(c, 1);
-	com_list_iter(c, n, CHILD(n, 0), tmpname);
+	com_list_for(c, CHILD(n, 1), CHILD(n, 0), tmpname);
 	com_addopnamestr(c, DELETE_NAME, tmpname);
 	--c->c_tmpname;
 }
@@ -1060,8 +1060,8 @@ com_list_comprehension(struct compiling *c, node *n)
 static void
 com_listmaker(struct compiling *c, node *n)
 {
-	/* listmaker: test ( list_iter | (',' test)* [','] ) */
-	if (NCH(n) > 1 && TYPE(CHILD(n, 1)) == list_iter)
+	/* listmaker: test ( list_for | (',' test)* [','] ) */
+	if (NCH(n) > 1 && TYPE(CHILD(n, 1)) == list_for)
 		com_list_comprehension(c, n);
 	else {
 		int len = 0;

Python/graminit.c

@@ -1317,7 +1317,7 @@ static state states_59[4] = {
 	{1, arcs_59_3},
 };
 static arc arcs_60_0[2] = {
-	{128, 1},
+	{120, 1},
 	{129, 1},
 };
 static arc arcs_60_1[1] = {
@@ -1340,7 +1340,7 @@ static arc arcs_61_3[1] = {
 	{9, 4},
 };
 static arc arcs_61_4[2] = {
-	{120, 5},
+	{128, 5},
 	{0, 4},
 };
 static arc arcs_61_5[1] = {
@@ -1361,7 +1361,7 @@ static arc arcs_62_1[1] = {
 	{21, 2},
 };
 static arc arcs_62_2[2] = {
-	{120, 3},
+	{128, 3},
 	{0, 2},
 };
 static arc arcs_62_3[1] = {
@@ -1622,7 +1622,7 @@ static label labels[130] = {
 	{25, 0},
 	{2, 0},
 	{3, 0},
-	{316, 0},
+	{317, 0},
 	{1, "lambda"},
 	{314, 0},
 	{307, 0},
@@ -1630,7 +1630,7 @@ static label labels[130] = {
 	{309, 0},
 	{1, "class"},
 	{315, 0},
-	{317, 0},
+	{316, 0},
 	{318, 0},
 };
 grammar _PyParser_Grammar = {