We are currently solving the following issue within our repository. Here is the issue text:
--- BEGIN ISSUE ---
ParamResolver does not have evaluatable repr
The reason is that values can be resolved into instances of sympy.Basic. For instance, `repr(cirq.ParamResolver({'a': sympy.Symbol('b') + 1}))` evaluates to `"cirq.ParamResolver({'a': b + 1})"` where the `b` is not a defined expression.

This is currently blocking JSON serialization of ParamResolver because the serialization test insists that the repr be evaluatable. Should we drop this requirement for ParamResolver? Or should we actually try to get an evaluatable repr for arbitrary sympy.Basic instances? As @mpharrigan mentioned in #2014 we could maybe use `sympy.srepr`.

@Strilanc 

--- END ISSUE ---

Below are some code segments, each from a relevant file. One or more of these files may contain bugs.
--- BEGIN FILES ---
Path: `cirq/study/resolver.py`
Content:
```
1 # Copyright 2018 The Cirq Developers
2 #
3 # Licensed under the Apache License, Version 2.0 (the "License");
4 # you may not use this file except in compliance with the License.
5 # You may obtain a copy of the License at
6 #
7 #     https://www.apache.org/licenses/LICENSE-2.0
8 #
9 # Unless required by applicable law or agreed to in writing, software
10 # distributed under the License is distributed on an "AS IS" BASIS,
11 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
12 # See the License for the specific language governing permissions and
13 # limitations under the License.
14 
15 """Resolves ParameterValues to assigned values."""
16 
17 from typing import Dict, Union, TYPE_CHECKING, cast
18 import sympy
19 from cirq._doc import document
20 
21 if TYPE_CHECKING:
22     import cirq
23 
24 
25 ParamDictType = Dict[Union[str, sympy.Symbol], Union[float, str, sympy.Basic]]
26 document(
27     ParamDictType,  # type: ignore
28     """Dictionary from symbols to values.""")
29 
30 ParamResolverOrSimilarType = Union['cirq.ParamResolver', ParamDictType, None]
31 document(
32     ParamResolverOrSimilarType,  # type: ignore
33     """Something that can be used to turn parameters into values.""")
34 
35 
36 class ParamResolver(object):
37     """Resolves sympy.Symbols to actual values.
38 
39     A Symbol is a wrapped parameter name (str). A ParamResolver is an object
40     that can be used to assign values for these keys.
41 
42     ParamResolvers are hashable.
43 
44     Attributes:
45         param_dict: A dictionary from the ParameterValue key (str) to its
46             assigned value.
47     """
48 
49     def __new__(cls, param_dict: 'cirq.ParamResolverOrSimilarType' = None):
50         if isinstance(param_dict, ParamResolver):
51             return param_dict
52         return super().__new__(cls)
53 
54     def __init__(self,
55                  param_dict: 'cirq.ParamResolverOrSimilarType' = None) -> None:
56         if hasattr(self, 'param_dict'):
57             return  # Already initialized. Got wrapped as part of the __new__.
58 
59         self._param_hash = None
60         self.param_dict = cast(ParamDictType,
61                                {} if param_dict is None else param_dict)
62 
63     def value_of(self,
64                  value: Union[sympy.Basic, float, str]) -> 'cirq.TParamVal':
65         """Attempt to resolve a Symbol, string, or float to its assigned value.
66 
67         Floats are returned without modification.  Strings are resolved via
68         the parameter dictionary with exact match only.  Otherwise, strings
69         are considered to be sympy.Symbols with the name as the input string.
70 
71         sympy.Symbols are first checked for exact match in the parameter
72         dictionary.  Otherwise, the symbol is resolved using sympy substitution.
73 
74         Note that passing a formula to this resolver can be slow due to the
75         underlying sympy library.  For circuits relying on quick performance,
76         it is recommended that all formulas are flattened before-hand using
77         cirq.flatten or other means so that formula resolution is avoided.
78         If unable to resolve a sympy.Symbol, returns it unchanged.
79         If unable to resolve a name, returns a sympy.Symbol with that name.
80 
81         Args:
82             value: The sympy.Symbol or name or float to try to resolve into just
83                 a float.
84 
85         Returns:
86             The value of the parameter as resolved by this resolver.
87         """
88         # Input is a float, no resolution needed: return early
89         if isinstance(value, float):
90             return value
91 
92         # Handles 2 cases:
93         # Input is a string and maps to a number in the dictionary
94         # Input is a symbol and maps to a number in the dictionary
95         # In both cases, return it directly.
96         if value in self.param_dict:
97             param_value = self.param_dict[value]
98             if isinstance(param_value, (float, int)):
99                 return param_value
100 
101         # Input is a string and is not in the dictionary.
102         # Treat it as a symbol instead.
103         if isinstance(value, str):
104             # If the string is in the param_dict as a value, return it.
105             # Otherwise, try using the symbol instead.
106             return self.value_of(sympy.Symbol(value))
107 
108         # Input is a symbol (sympy.Symbol('a')) and its string maps to a number
109         # in the dictionary ({'a': 1.0}).  Return it.
110         if (isinstance(value, sympy.Symbol) and value.name in self.param_dict):
111             param_value = self.param_dict[value.name]
112             if isinstance(param_value, (float, int)):
113                 return param_value
114 
115         # Input is either a sympy formula or the dictionary maps to a
116         # formula.  Use sympy to resolve the value.
117         # Note that sympy.subs() is slow, so we want to avoid this and
118         # only use it for cases that require complicated resolution.
119         if isinstance(value, sympy.Basic):
120             v = value.subs(self.param_dict)
121             if v.free_symbols:
122                 return v
123             elif sympy.im(v):
124                 return complex(v)
125             else:
126                 return float(v)
127 
128         # No known way to resolve this variable, return unchanged.
129         return value
130 
131     def __iter__(self):
132         return iter(self.param_dict)
133 
134     def __bool__(self):
135         return bool(self.param_dict)
136 
137     def __getitem__(self, key):
138         return self.value_of(key)
139 
140     def __hash__(self):
141         if self._param_hash is None:
142             self._param_hash = hash(frozenset(self.param_dict.items()))
143         return self._param_hash
144 
145     def __eq__(self, other):
146         if not isinstance(other, ParamResolver):
147             return NotImplemented
148         return self.param_dict == other.param_dict
149 
150     def __ne__(self, other):
151         return not self == other
152 
153     def __repr__(self):
154         return 'cirq.ParamResolver({})'.format(repr(self.param_dict))
155
```
--- END FILES ---

Please first localize the bug based on the issue statement, and then generate a patch according to the `git diff` format fenced by three backticks.

Here is an example:

```diff
diff --git a/examples/server_async.py b/examples/server_async.py
--- a/examples/server_async.py
+++ b/examples/server_async.py
@@ -313,4 +313,4 @@
 
 
 if __name__ == "__main__":
-    asyncio.run(run_async_server("."), debug=True)
+    asyncio.run(run_async_server(), debug=True)
diff --git a/examples/server_sync.py b/examples/server_sync.py
--- a/examples/server_sync.py
+++ b/examples/server_sync.py
@@ -313,5 +313,5 @@
 
 
 if __name__ == "__main__":
-    server = run_sync_server(".")
+    server = run_sync_server()
     server.shutdown()

```

diff --git a/cirq/study/resolver.py b/cirq/study/resolver.py
--- a/cirq/study/resolver.py
+++ b/cirq/study/resolver.py
@@ -16,6 +16,7 @@
 
 from typing import Dict, Union, TYPE_CHECKING, cast
 import sympy
+from cirq._compat import proper_repr
 from cirq._doc import document
 
 if TYPE_CHECKING:
@@ -151,4 +152,8 @@
         return not self == other
 
     def __repr__(self):
-        return 'cirq.ParamResolver({})'.format(repr(self.param_dict))
+        param_dict_repr = ('{' + ', '.join([
+            f'{proper_repr(k)}: {proper_repr(v)}'
+            for k, v in self.param_dict.items()
+        ]) + '}')
+        return 'cirq.ParamResolver({})'.format(param_dict_repr)

{"golden_diff": "diff --git a/cirq/study/resolver.py b/cirq/study/resolver.py\n--- a/cirq/study/resolver.py\n+++ b/cirq/study/resolver.py\n@@ -16,6 +16,7 @@\n \n from typing import Dict, Union, TYPE_CHECKING, cast\n import sympy\n+from cirq._compat import proper_repr\n from cirq._doc import document\n \n if TYPE_CHECKING:\n@@ -151,4 +152,8 @@\n         return not self == other\n \n     def __repr__(self):\n-        return 'cirq.ParamResolver({})'.format(repr(self.param_dict))\n+        param_dict_repr = ('{' + ', '.join([\n+            f'{proper_repr(k)}: {proper_repr(v)}'\n+            for k, v in self.param_dict.items()\n+        ]) + '}')\n+        return 'cirq.ParamResolver({})'.format(param_dict_repr)\n", "issue": "ParamResolver does not have evaluatable repr\nThe reason is that values can be resolved into instances of sympy.Basic. For instance, `repr(cirq.ParamResolver({'a': sympy.Symbol('b') + 1}))` evaluates to `\"cirq.ParamResolver({'a': b + 1})\"` where the `b` is not a defined expression.\r\n\r\nThis is currently blocking JSON serialization of ParamResolver because the serialization test insists that the repr be evaluatable. Should we drop this requirement for ParamResolver? Or should we actually try to get an evaluatable repr for arbitrary sympy.Basic instances? As @mpharrigan mentioned in #2014 we could maybe use `sympy.srepr`.\r\n\r\n@Strilanc \n", "before_files": [{"content": "# Copyright 2018 The Cirq Developers\n#\n# Licensed under the Apache License, Version 2.0 (the \"License\");\n# you may not use this file except in compliance with the License.\n# You may obtain a copy of the License at\n#\n#     https://www.apache.org/licenses/LICENSE-2.0\n#\n# Unless required by applicable law or agreed to in writing, software\n# distributed under the License is distributed on an \"AS IS\" BASIS,\n# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.\n# See the License for the specific language governing permissions and\n# limitations under the License.\n\n\"\"\"Resolves ParameterValues to assigned values.\"\"\"\n\nfrom typing import Dict, Union, TYPE_CHECKING, cast\nimport sympy\nfrom cirq._doc import document\n\nif TYPE_CHECKING:\n    import cirq\n\n\nParamDictType = Dict[Union[str, sympy.Symbol], Union[float, str, sympy.Basic]]\ndocument(\n    ParamDictType,  # type: ignore\n    \"\"\"Dictionary from symbols to values.\"\"\")\n\nParamResolverOrSimilarType = Union['cirq.ParamResolver', ParamDictType, None]\ndocument(\n    ParamResolverOrSimilarType,  # type: ignore\n    \"\"\"Something that can be used to turn parameters into values.\"\"\")\n\n\nclass ParamResolver(object):\n    \"\"\"Resolves sympy.Symbols to actual values.\n\n    A Symbol is a wrapped parameter name (str). A ParamResolver is an object\n    that can be used to assign values for these keys.\n\n    ParamResolvers are hashable.\n\n    Attributes:\n        param_dict: A dictionary from the ParameterValue key (str) to its\n            assigned value.\n    \"\"\"\n\n    def __new__(cls, param_dict: 'cirq.ParamResolverOrSimilarType' = None):\n        if isinstance(param_dict, ParamResolver):\n            return param_dict\n        return super().__new__(cls)\n\n    def __init__(self,\n                 param_dict: 'cirq.ParamResolverOrSimilarType' = None) -> None:\n        if hasattr(self, 'param_dict'):\n            return  # Already initialized. Got wrapped as part of the __new__.\n\n        self._param_hash = None\n        self.param_dict = cast(ParamDictType,\n                               {} if param_dict is None else param_dict)\n\n    def value_of(self,\n                 value: Union[sympy.Basic, float, str]) -> 'cirq.TParamVal':\n        \"\"\"Attempt to resolve a Symbol, string, or float to its assigned value.\n\n        Floats are returned without modification.  Strings are resolved via\n        the parameter dictionary with exact match only.  Otherwise, strings\n        are considered to be sympy.Symbols with the name as the input string.\n\n        sympy.Symbols are first checked for exact match in the parameter\n        dictionary.  Otherwise, the symbol is resolved using sympy substitution.\n\n        Note that passing a formula to this resolver can be slow due to the\n        underlying sympy library.  For circuits relying on quick performance,\n        it is recommended that all formulas are flattened before-hand using\n        cirq.flatten or other means so that formula resolution is avoided.\n        If unable to resolve a sympy.Symbol, returns it unchanged.\n        If unable to resolve a name, returns a sympy.Symbol with that name.\n\n        Args:\n            value: The sympy.Symbol or name or float to try to resolve into just\n                a float.\n\n        Returns:\n            The value of the parameter as resolved by this resolver.\n        \"\"\"\n        # Input is a float, no resolution needed: return early\n        if isinstance(value, float):\n            return value\n\n        # Handles 2 cases:\n        # Input is a string and maps to a number in the dictionary\n        # Input is a symbol and maps to a number in the dictionary\n        # In both cases, return it directly.\n        if value in self.param_dict:\n            param_value = self.param_dict[value]\n            if isinstance(param_value, (float, int)):\n                return param_value\n\n        # Input is a string and is not in the dictionary.\n        # Treat it as a symbol instead.\n        if isinstance(value, str):\n            # If the string is in the param_dict as a value, return it.\n            # Otherwise, try using the symbol instead.\n            return self.value_of(sympy.Symbol(value))\n\n        # Input is a symbol (sympy.Symbol('a')) and its string maps to a number\n        # in the dictionary ({'a': 1.0}).  Return it.\n        if (isinstance(value, sympy.Symbol) and value.name in self.param_dict):\n            param_value = self.param_dict[value.name]\n            if isinstance(param_value, (float, int)):\n                return param_value\n\n        # Input is either a sympy formula or the dictionary maps to a\n        # formula.  Use sympy to resolve the value.\n        # Note that sympy.subs() is slow, so we want to avoid this and\n        # only use it for cases that require complicated resolution.\n        if isinstance(value, sympy.Basic):\n            v = value.subs(self.param_dict)\n            if v.free_symbols:\n                return v\n            elif sympy.im(v):\n                return complex(v)\n            else:\n                return float(v)\n\n        # No known way to resolve this variable, return unchanged.\n        return value\n\n    def __iter__(self):\n        return iter(self.param_dict)\n\n    def __bool__(self):\n        return bool(self.param_dict)\n\n    def __getitem__(self, key):\n        return self.value_of(key)\n\n    def __hash__(self):\n        if self._param_hash is None:\n            self._param_hash = hash(frozenset(self.param_dict.items()))\n        return self._param_hash\n\n    def __eq__(self, other):\n        if not isinstance(other, ParamResolver):\n            return NotImplemented\n        return self.param_dict == other.param_dict\n\n    def __ne__(self, other):\n        return not self == other\n\n    def __repr__(self):\n        return 'cirq.ParamResolver({})'.format(repr(self.param_dict))\n", "path": "cirq/study/resolver.py"}], "after_files": [{"content": "# Copyright 2018 The Cirq Developers\n#\n# Licensed under the Apache License, Version 2.0 (the \"License\");\n# you may not use this file except in compliance with the License.\n# You may obtain a copy of the License at\n#\n#     https://www.apache.org/licenses/LICENSE-2.0\n#\n# Unless required by applicable law or agreed to in writing, software\n# distributed under the License is distributed on an \"AS IS\" BASIS,\n# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.\n# See the License for the specific language governing permissions and\n# limitations under the License.\n\n\"\"\"Resolves ParameterValues to assigned values.\"\"\"\n\nfrom typing import Dict, Union, TYPE_CHECKING, cast\nimport sympy\nfrom cirq._compat import proper_repr\nfrom cirq._doc import document\n\nif TYPE_CHECKING:\n    import cirq\n\n\nParamDictType = Dict[Union[str, sympy.Symbol], Union[float, str, sympy.Basic]]\ndocument(\n    ParamDictType,  # type: ignore\n    \"\"\"Dictionary from symbols to values.\"\"\")\n\nParamResolverOrSimilarType = Union['cirq.ParamResolver', ParamDictType, None]\ndocument(\n    ParamResolverOrSimilarType,  # type: ignore\n    \"\"\"Something that can be used to turn parameters into values.\"\"\")\n\n\nclass ParamResolver(object):\n    \"\"\"Resolves sympy.Symbols to actual values.\n\n    A Symbol is a wrapped parameter name (str). A ParamResolver is an object\n    that can be used to assign values for these keys.\n\n    ParamResolvers are hashable.\n\n    Attributes:\n        param_dict: A dictionary from the ParameterValue key (str) to its\n            assigned value.\n    \"\"\"\n\n    def __new__(cls, param_dict: 'cirq.ParamResolverOrSimilarType' = None):\n        if isinstance(param_dict, ParamResolver):\n            return param_dict\n        return super().__new__(cls)\n\n    def __init__(self,\n                 param_dict: 'cirq.ParamResolverOrSimilarType' = None) -> None:\n        if hasattr(self, 'param_dict'):\n            return  # Already initialized. Got wrapped as part of the __new__.\n\n        self._param_hash = None\n        self.param_dict = cast(ParamDictType,\n                               {} if param_dict is None else param_dict)\n\n    def value_of(self,\n                 value: Union[sympy.Basic, float, str]) -> 'cirq.TParamVal':\n        \"\"\"Attempt to resolve a Symbol, string, or float to its assigned value.\n\n        Floats are returned without modification.  Strings are resolved via\n        the parameter dictionary with exact match only.  Otherwise, strings\n        are considered to be sympy.Symbols with the name as the input string.\n\n        sympy.Symbols are first checked for exact match in the parameter\n        dictionary.  Otherwise, the symbol is resolved using sympy substitution.\n\n        Note that passing a formula to this resolver can be slow due to the\n        underlying sympy library.  For circuits relying on quick performance,\n        it is recommended that all formulas are flattened before-hand using\n        cirq.flatten or other means so that formula resolution is avoided.\n        If unable to resolve a sympy.Symbol, returns it unchanged.\n        If unable to resolve a name, returns a sympy.Symbol with that name.\n\n        Args:\n            value: The sympy.Symbol or name or float to try to resolve into just\n                a float.\n\n        Returns:\n            The value of the parameter as resolved by this resolver.\n        \"\"\"\n        # Input is a float, no resolution needed: return early\n        if isinstance(value, float):\n            return value\n\n        # Handles 2 cases:\n        # Input is a string and maps to a number in the dictionary\n        # Input is a symbol and maps to a number in the dictionary\n        # In both cases, return it directly.\n        if value in self.param_dict:\n            param_value = self.param_dict[value]\n            if isinstance(param_value, (float, int)):\n                return param_value\n\n        # Input is a string and is not in the dictionary.\n        # Treat it as a symbol instead.\n        if isinstance(value, str):\n            # If the string is in the param_dict as a value, return it.\n            # Otherwise, try using the symbol instead.\n            return self.value_of(sympy.Symbol(value))\n\n        # Input is a symbol (sympy.Symbol('a')) and its string maps to a number\n        # in the dictionary ({'a': 1.0}).  Return it.\n        if (isinstance(value, sympy.Symbol) and value.name in self.param_dict):\n            param_value = self.param_dict[value.name]\n            if isinstance(param_value, (float, int)):\n                return param_value\n\n        # Input is either a sympy formula or the dictionary maps to a\n        # formula.  Use sympy to resolve the value.\n        # Note that sympy.subs() is slow, so we want to avoid this and\n        # only use it for cases that require complicated resolution.\n        if isinstance(value, sympy.Basic):\n            v = value.subs(self.param_dict)\n            if v.free_symbols:\n                return v\n            elif sympy.im(v):\n                return complex(v)\n            else:\n                return float(v)\n\n        # No known way to resolve this variable, return unchanged.\n        return value\n\n    def __iter__(self):\n        return iter(self.param_dict)\n\n    def __bool__(self):\n        return bool(self.param_dict)\n\n    def __getitem__(self, key):\n        return self.value_of(key)\n\n    def __hash__(self):\n        if self._param_hash is None:\n            self._param_hash = hash(frozenset(self.param_dict.items()))\n        return self._param_hash\n\n    def __eq__(self, other):\n        if not isinstance(other, ParamResolver):\n            return NotImplemented\n        return self.param_dict == other.param_dict\n\n    def __ne__(self, other):\n        return not self == other\n\n    def __repr__(self):\n        param_dict_repr = ('{' + ', '.join([\n            f'{proper_repr(k)}: {proper_repr(v)}'\n            for k, v in self.param_dict.items()\n        ]) + '}')\n        return 'cirq.ParamResolver({})'.format(param_dict_repr)\n", "path": "cirq/study/resolver.py"}]}