Watch out for these possible mistakes:
1.Kill and reopen your shell window: Once you make a change to the ENVIRONMENTAL Variables, you have to restart the window you are testing it on.
2.NO SPACES when setting the Variables. Make sure that you are adding the ;C:\Python27 WITHOUT any spaces. (It is common to try C:\SomeOther; C:\Python27 That space (_) after the semicolon is not okay.)
3.USE A BACKWARD SLASH when spelling out your full path. You will see forward slashes when you try echo $PATH but only forward slashes have worked for me.
4.DO NOT ADD a final backslash. Only C:\Python27 NOT C:\Python27\

1.添加环境变量时不要带空格。例如: C:\SomeOther; C:\Python27 分号后面不能有空格。正确形式:C:\SomeOther;C:\Python27
3.路径最后不要加反斜线。使用 C:\Python27 而不是 C:\Python27\