[diffoscope] 01/02: presenters: PartialString: preserve {{ and }} escapes when partially formatting stuff

[Ximin Luo]

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infinity0 pushed a commit to branch WIP/humungous-diffs
in repository diffoscope.

commit 7a870837881d629a81788091c05431756179afa3
Author: Ximin Luo <infinity0 at debian.org>
Date:   Mon Jun 19 18:02:40 2017 +0200

    presenters: PartialString: preserve {{ and }} escapes when partially formatting stuff
---
 diffoscope/presenters/utils.py | 31 +++++++++++++++++++++----------
 tests/test_presenters.py       |  7 +++++++
 2 files changed, 28 insertions(+), 10 deletions(-)

diff --git a/diffoscope/presenters/utils.py b/diffoscope/presenters/utils.py
index 69040d6..c46773a 100644
--- a/diffoscope/presenters/utils.py
+++ b/diffoscope/presenters/utils.py
@@ -104,12 +104,18 @@ def create_limited_print_func(print_func, max_page_size):
     return fn
 
 
-class Formatter(string.Formatter):
+class PartialFormatter(string.Formatter):
+    @staticmethod
+    def escape(x):
+        return x.replace("}", "}}").replace("{", "{{")
     def get_value(self, key, args, kwargs):
         return args[key] if isinstance(key, int) else args[int(key)]
     def arg_of_field_name(self, field_name, args):
         x = int(_string.formatter_field_name_split(field_name)[0])
         return x if x >= 0 else len(args) + x
+    def parse(self, *args, **kwargs):
+        # Preserve {{ and }} escapes when formatting
+        return map(lambda x: (self.escape(x[0]),) + x[1:], super().parse(*args, **kwargs))
 
 
 class FormatPlaceholder(object):
@@ -192,7 +198,8 @@ class PartialString(object):
     So you probably want to avoid such usages. The exact behaviour of these
     might change in the future, too.
     """
-    formatter = Formatter()
+    formatter = PartialFormatter()
+    escape = staticmethod(PartialFormatter.escape)
 
     def __init__(self, fmtstr="", *holes):
         # Ensure the format string is valid, and figure out some basic stats
@@ -217,8 +224,12 @@ class PartialString(object):
     def __repr__(self):
         return "%s%r" % (self.__class__.__name__, (self._fmtstr,) + self.holes)
 
+    def _format(self, *mapping):
+        # format a string but preserve {{ and }} escapes
+        return self.formatter.vformat(self._fmtstr, mapping, None)
+
     def _offset_fmtstr(self, offset):
-        return self._fmtstr.format(*(FormatPlaceholder(i + offset) for i in range(len(self.holes))))
+        return self._format(*(FormatPlaceholder(i + offset) for i in range(len(self.holes))))
 
     def _pformat(self, mapping):
         new_holes = []
@@ -235,22 +246,22 @@ class PartialString(object):
                 out = FormatPlaceholder(len(new_holes))
                 new_holes.append(k)
             real_mapping.append(out)
-        return self._fmtstr.format(*real_mapping), new_holes
+        return real_mapping, new_holes
 
     def size(self, hole_size=1):
         return self.base_len + hole_size * self.num_holes
 
-    def pformat(self, mapping):
+    def pformat(self, mapping={}):
         """Partially apply a mapping, returning a new PartialString."""
-        new_fmtstr, new_holes = self._pformat(mapping)
-        return self.__class__(new_fmtstr, *new_holes)
+        real_mapping, new_holes = self._pformat(mapping)
+        return self.__class__(self._format(*real_mapping), *new_holes)
 
-    def format(self, mapping):
+    def format(self, mapping={}):
         """Fully apply a mapping, returning a string."""
-        new_fmtstr, new_holes = self._pformat(mapping)
+        real_mapping, new_holes = self._pformat(mapping)
         if new_holes:
             raise ValueError("not all holes filled: %r" % new_holes)
-        return new_fmtstr
+        return self._fmtstr.format(*real_mapping)
 
     @classmethod
     def of(cls, obj):
diff --git a/tests/test_presenters.py b/tests/test_presenters.py
index 62b3ac2..d8cfd5c 100644
--- a/tests/test_presenters.py
+++ b/tests/test_presenters.py
@@ -177,3 +177,10 @@ def test_partial_string():
     assert (t.format({key[0]: "line1", key[1]: "line2", key[2]: "line3"})
             == 'x: line1\ny: line2\nz: line3\n')
     assert t.size(hole_size=5) == 27
+
+def test_partial_string_escape():
+    esc = PartialString("{{}} {0}", None)
+    assert esc.pformat({None: PartialString.of(None)}) == esc
+    assert esc.format({None: "0"}) == "{} 0"
+    with pytest.raises(ValueError):
+        PartialString("{}")

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