在許多情況下，環境變數的用途和適用性可能不會立刻顯現。但是在開發過程中，它們會在許多不同的場景中出現，因此瞭解它們是非常必要的。

   *
   * @param formal The type whose type variables or itself is mapped to other type(s). It's almost
   *     always a bug if {@code formal} isn't a type variable and contains no type variable. Make
   *     sure you are passing the two parameters in the right order.
   * @param actual The type that the formal type variable(s) are mapped to. It can be or contain yet

# Biến môi trường (Environment Variables)

/// tip

Nếu bạn đã biết về "biến môi trường" và cách sử dụng chúng, bạn có thể bỏ qua phần này.

///

Một biến môi trường (còn được gọi là "**env var**") là một biến mà tồn tại **bên ngoài** đoạn mã Python, ở trong **hệ điều hành**, và có thể được đọc bởi đoạn mã Python của bạn (hoặc bởi các chương trình khác).

   *
   * @param formal The type whose type variables or itself is mapped to other type(s). It's almost
   *     always a bug if {@code formal} isn't a type variable and contains no type variable. Make
   *     sure you are passing the two parameters in the right order.
   * @param actual The type that the formal type variable(s) are mapped to. It can be or contain yet

#### Liste

Definieren wir zum Beispiel eine Variable, die eine `list` von `str` – eine Liste von Strings – sein soll.

//// tab | Python 3.9+

Deklarieren Sie die Variable mit der gleichen Doppelpunkt-Syntax (`:`).

Als Typ nehmen Sie `list`.

#### List

Laten we bijvoorbeeld een variabele definiëren als een `list` van `str`.

//// tab | Python 3.9+

Declareer de variabele met dezelfde dubbele punt (`:`) syntax.

Als type, vul `list` in.

Doordat de list een type is dat enkele interne types bevat, zet je ze tussen vierkante haakjes:

   * add the suppression to the whole method (which turns off checking for a large section of code),
   * or extract a variable, and put the suppression on that. However, a local variable typically
   * doesn't work: Because nullness analyses typically infer the nullness of local variables,
   * there's no way to assign a {@code @Nullable T} to a field {@code T foo;} and instruct the

     * <li>If <code>type</code> is a wildcard type, its (first) upper bound is returned.</li>
     * <li>If <code>type</code> is a type variable and is a key in the given map, its actual type argument is returned.</li>
     * <li>If <code>type</code> is a type variable and is not a key in the given map, its (first) upper bound is returned.</li>
     * <li>If <code>type</code> is an array, the actual class of its elements is returned.</li>

:end
@rem End local scope for the variables with windows NT shell
if %ERRORLEVEL% equ 0 goto mainEnd

:fail
rem Set variable GRADLE_EXIT_CONSOLE if you need the _script_ return code instead of
rem the _cmd.exe /c_ return code!
set EXIT_CODE=%ERRORLEVEL%
if %EXIT_CODE% equ 0 set EXIT_CODE=1
if not ""=="%GRADLE_EXIT_CONSOLE%" exit %EXIT_CODE%
exit /b %EXIT_CODE%

:mainEnd

:end
@rem End local scope for the variables with windows NT shell
if %ERRORLEVEL% equ 0 goto mainEnd

:fail
rem Set variable GRADLE_EXIT_CONSOLE if you need the _script_ return code instead of
rem the _cmd.exe /c_ return code!
set EXIT_CODE=%ERRORLEVEL%
if %EXIT_CODE% equ 0 set EXIT_CODE=1
if not ""=="%GRADLE_EXIT_CONSOLE%" exit %EXIT_CODE%
exit /b %EXIT_CODE%

:mainEnd