TYPE_CHECKING_tutorial

Author: twtrubiks

type-hints-tutorial/README.md

@@ -103,6 +103,8 @@ A compromise is possible where a __future__ import could enable turning all anno
 
 - [Protocol](https://github.com/twtrubiks/python-notes/blob/master/type-hints-tutorial/Protocol_tutorial.py)
 
+- [TYPE_CHECKING](https://github.com/twtrubiks/python-notes/tree/master/type-hints-tutorial/TYPE_CHECKING_tutorial) - 說明 TYPE_CHECKING 用法
+
 ### Type comments
 
 ```python

type-hints-tutorial/TYPE_CHECKING_tutorial/README.md

@@ -0,0 +1,72 @@
+# TYPE_CHECKING 介紹
+
+說明一下 TYPE_CHECKING 的功能, 主要功能是避免 python 循環引用以及讓 IDE 支援靜態檢查.
+
+```python
+from typing import TYPE_CHECKING
+```
+
+先來看一個例子 [module_a.py](module_a.py)
+
+```python
+from module_b import ClassB
+
+class ClassA:
+    def __init__(self, b: ClassB) -> None:
+        self.b = b
+
+    def use_b(self) -> None:
+        print("Using ClassB from ClassA")
+```
+
+[module_b.py](module_b.py)
+
+```python
+from module_a import ClassA
+
+class ClassB:
+    def __init__(self, a: ClassA) -> None:
+        self.a = a
+
+    def use_a(self) -> None:
+        print("Using ClassA from ClassB")
+```
+
+當你執行 [main.py](main.py) 的時候, 會出現循環引用
+
+```python
+from module_a import ClassA
+from module_b import ClassB
+
+a = ClassA(None)
+b = ClassB(a)
+b.use_a()
+```
+
+那我們應該如何避免循環引用以及同時有 typing hint 的檢查呢 ❓
+
+這時候介紹 TYPE_CHECKING
+
+[module_a_fix.py](module_a_fix.py)
+
+```python
+from typing import TYPE_CHECKING
+
+if TYPE_CHECKING:
+    from module_b_fix import ClassB
+
+class ClassA:
+    def __init__(self, b: "ClassB") -> None:
+        self.b = b
+
+    def use_b(self) -> None:
+        print("Using ClassB from ClassA")
+```
+
+注意修改的地方, 首先, `TYPE_CHECKING = False` 預設是 False,
+
+只有在 IDE 的部份才會是 True, 然後需要把 ClassB 加上雙引號變成 `"ClassB"`,
+
+同理, [module_b_fix.py](module_b_fix.py) 一樣的修正方式.
+
+修改完後的 [main_fix.py](main_fix.py) 就可以正常的執行.

type-hints-tutorial/TYPE_CHECKING_tutorial/main.py

@@ -0,0 +1,6 @@
+from module_a import ClassA
+from module_b import ClassB
+
+a = ClassA(None)
+b = ClassB(a)
+b.use_a()

type-hints-tutorial/TYPE_CHECKING_tutorial/main_fix.py

@@ -0,0 +1,6 @@
+from module_a_fix import ClassA
+from module_b_fix import ClassB
+
+a = ClassA(None)
+b = ClassB(a)
+b.use_a()

type-hints-tutorial/TYPE_CHECKING_tutorial/module_a.py

@@ -0,0 +1,8 @@
+from module_b import ClassB
+
+class ClassA:
+    def __init__(self, b: ClassB) -> None:
+        self.b = b
+
+    def use_b(self) -> None:
+        print("Using ClassB from ClassA")

type-hints-tutorial/TYPE_CHECKING_tutorial/module_a_fix.py

@@ -0,0 +1,11 @@
+from typing import TYPE_CHECKING
+
+if TYPE_CHECKING:
+    from module_b_fix import ClassB
+
+class ClassA:
+    def __init__(self, b: "ClassB") -> None:
+        self.b = b
+
+    def use_b(self) -> None:
+        print("Using ClassB from ClassA")

type-hints-tutorial/TYPE_CHECKING_tutorial/module_b.py

@@ -0,0 +1,8 @@
+from module_a import ClassA
+
+class ClassB:
+    def __init__(self, a: ClassA) -> None:
+        self.a = a
+
+    def use_a(self) -> None:
+        print("Using ClassA from ClassB")

type-hints-tutorial/TYPE_CHECKING_tutorial/module_b_fix.py

@@ -0,0 +1,11 @@
+from typing import TYPE_CHECKING
+
+if TYPE_CHECKING:
+    from module_a_fix import ClassA
+
+class ClassB:
+    def __init__(self, a: "ClassA") -> None:
+        self.a = a
+
+    def use_a(self) -> None:
+        print("Using ClassA from ClassB")