/Lib/lib2to3/fixes/fix_except.py

 1"""Fixer for except statements with named exceptions.
 2
 3The following cases will be converted:
 4
 5- "except E, T:" where T is a name:
 6
 7    except E as T:
 8
 9- "except E, T:" where T is not a name, tuple or list:
10
11        except E as t:
12            T = t
13
14    This is done because the target of an "except" clause must be a
15    name.
16
17- "except E, T:" where T is a tuple or list literal:
18
19        except E as t:
20            T = t.args
21"""
22# Author: Collin Winter
23
24# Local imports
25from .. import pytree
26from ..pgen2 import token
27from .. import fixer_base
28from ..fixer_util import Assign, Attr, Name, is_tuple, is_list
29
30def find_excepts(nodes):
31    for i, n in enumerate(nodes):
32        if isinstance(n, pytree.Node):
33            if n.children[0].value == 'except':
34                yield (n, nodes[i+2])
35
36class FixExcept(fixer_base.BaseFix):
37
38    PATTERN = """
39    try_stmt< 'try' ':' suite
40                  cleanup=(except_clause ':' suite)+
41                  tail=(['except' ':' suite]
42                        ['else' ':' suite]
43                        ['finally' ':' suite]) >
44    """
45
46    def transform(self, node, results):
47        syms = self.syms
48
49        tail = [n.clone() for n in results["tail"]]
50
51        try_cleanup = [ch.clone() for ch in results["cleanup"]]
52        for except_clause, e_suite in find_excepts(try_cleanup):
53            if len(except_clause.children) == 4:
54                (E, comma, N) = except_clause.children[1:4]
55                comma.replace(Name("as", prefix=" "))
56
57                if N.type != token.NAME:
58                    # Generate a new N for the except clause
59                    new_N = Name(self.new_name(), prefix=" ")
60                    target = N.clone()
61                    target.set_prefix("")
62                    N.replace(new_N)
63                    new_N = new_N.clone()
64
65                    # Insert "old_N = new_N" as the first statement in
66                    #  the except body. This loop skips leading whitespace
67                    #  and indents
68                    #TODO(cwinter) suite-cleanup
69                    suite_stmts = e_suite.children
70                    for i, stmt in enumerate(suite_stmts):
71                        if isinstance(stmt, pytree.Node):
72                            break
73
74                    # The assignment is different if old_N is a tuple or list
75                    # In that case, the assignment is old_N = new_N.args
76                    if is_tuple(N) or is_list(N):
77                        assign = Assign(target, Attr(new_N, Name('args')))
78                    else:
79                        assign = Assign(target, new_N)
80
81                    #TODO(cwinter) stopgap until children becomes a smart list
82                    for child in reversed(suite_stmts[:i]):
83                        e_suite.insert_child(0, child)
84                    e_suite.insert_child(i, assign)
85                elif N.get_prefix() == "":
86                    # No space after a comma is legal; no space after "as",
87                    # not so much.
88                    N.set_prefix(" ")
89
90        #TODO(cwinter) fix this when children becomes a smart list
91        children = [c.clone() for c in node.children[:3]] + try_cleanup + tail
92        return pytree.Node(node.type, children)