Omit no-member error if guarded behind if stmt

ChangeLog

@@ -26,11 +26,18 @@ Release date: TBA
 
   Close #4120
 
+* Don't emit ``no-member`` error if guarded behind if statement.
+
+  Ref #1162
+  Closes #1990
+  Closes #4168
+
 * The default for ``PYLINTHOME`` is now the standard ``XDG_CACHE_HOME``, and pylint now uses ``appdirs``.
 
   Closes #3878
 
 
+
 What's New in Pylint 2.9.6?
 ===========================
 Release date: 2021-07-28

doc/whatsnew/2.10.rst

@@ -24,5 +24,12 @@ Other Changes
 * Performance of the Similarity checker has been improved.
 
 * Added ``time.clock`` to deprecated functions/methods for python 3.3
+
 * Added ``ignored-parents`` option to the design checker to ignore specific
   classes from the ``too-many-ancestors`` check (R0901).
+
+* Don't emit ``no-member`` error if guarded behind if statement.
+
+  Ref #1162
+  Closes #1990
+  Closes #4168

pylint/checkers/typecheck.py

@@ -454,6 +454,7 @@ def _emit_no_member(node, owner, owner_name, ignored_mixins=True, ignored_none=T
         * the owner is a class and the name can be found in its metaclass.
         * The access node is protected by an except handler, which handles
           AttributeError, Exception or bare except.
+        * The node is guarded behind and `IF` or `IFExp` node
     """
     # pylint: disable=too-many-return-statements
     if node_ignores_exception(node, AttributeError):
@@ -522,6 +523,30 @@ def _emit_no_member(node, owner, owner_name, ignored_mixins=True, ignored_none=T
         # Avoid false positive on Enum.__members__.{items(), values, keys}
         # See https://github.com/PyCQA/pylint/issues/4123
         return False
+    # Don't emit no-member if guarded behind `IF` or `IFExp`
+    #   * Walk up recursively until if statement is found.
+    #   * Check if condition can be inferred as `Const`,
+    #       would evaluate as `False`,
+    #       and wheater the node is part of the `body`.
+    #   * Continue checking until scope of node is reached.
+    scope: astroid.NodeNG = node.scope()
+    node_origin: astroid.NodeNG = node
+    parent: astroid.NodeNG = node.parent
+    while parent != scope:
+        if isinstance(parent, (astroid.If, astroid.IfExp)):
+            inferred = safe_infer(parent.test)
+            if (  # pylint: disable=too-many-boolean-expressions
+                isinstance(inferred, astroid.Const)
+                and inferred.bool_value() is False
+                and (
+                    isinstance(parent, astroid.If)
+                    and node_origin in parent.body
+                    or isinstance(parent, astroid.IfExp)
+                    and node_origin == parent.body
+                )
+            ):
+                return False
+        node_origin, parent = parent, parent.parent
 
     return True
 

tests/functional/n/no/no_member_if_statements.py

@@ -0,0 +1,79 @@
+# pylint: disable=invalid-name,missing-docstring,pointless-statement
+from datetime import datetime
+from typing import Union
+
+value = "Hello World"
+value.isoformat()  # [no-member]
+
+
+if isinstance(value, datetime):
+    value.isoformat()
+else:
+    value.isoformat()  # [no-member]
+
+
+def func():
+    if hasattr(value, "load"):
+        value.load()
+
+    if getattr(value, "load", None):
+        value.load
+
+
+if value.none_existent():  # [no-member]
+    pass
+
+res = value.isoformat() if isinstance(value, datetime) else value
+
+
+class Base:
+    _attr_state: Union[str, datetime] = "Unknown"
+
+    @property
+    def state(self) -> Union[str, datetime]:
+        return self._attr_state
+
+    def some_function(self) -> str:
+        state = self.state
+        if isinstance(state, datetime):
+            return state.isoformat()
+        return str(state)
+
+
+# https://github.com/PyCQA/pylint/issues/1990
+# Attribute access after 'isinstance' should not cause 'no-member' error
+import subprocess  # pylint: disable=wrong-import-position  # noqa: E402
+
+try:
+    subprocess.check_call(['ls', '-'])  # Deliberately made error in this line
+except Exception as err:
+    if isinstance(err, subprocess.CalledProcessError):
+        print(f'Subprocess error occured. Return code: {err.returncode}')
+    else:
+        print(f'An error occured: {str(err)}')
+    raise
+
+
+# https://github.com/PyCQA/pylint/issues/4168
+# 'encode' for 'arg' should not cause 'no-member' error
+mixed_tuple = (b"a", b"b", b"c", b"d")
+byte_tuple = [arg.encode('utf8') if isinstance(arg, str) else arg for arg in mixed_tuple]
+
+for arg in mixed_tuple:
+    if isinstance(arg, str):
+        print(arg.encode('utf8'))
+    else:
+        print(arg)
+
+
+# https://github.com/PyCQA/pylint/issues/1162
+# Attribute access after 'isinstance' should not cause 'no-member' error
+class FoobarException(Exception):
+    foobar = None
+
+try:  # noqa: E305
+    pass
+except Exception as ex:
+    if isinstance(ex, FoobarException):
+        ex.foobar
+    raise

tests/functional/n/no/no_member_if_statements.txt

@@ -0,0 +1,3 @@
+no-member:6:0::Instance of 'str' has no 'isoformat' member:INFERENCE
+no-member:12:4::Instance of 'str' has no 'isoformat' member:INFERENCE
+no-member:23:3::Instance of 'str' has no 'none_existent' member:INFERENCE