python 安装的问题

Question

chins-MBP:~chinvincent$pipinstallrequestsCollectingrequestsUsingcachedhttps://files.pythonhosted.org/packages/49/df/50aa1999ab9bde74656c2919d9c0c085fd2b3775fd3eca826012bef76d8c/requests-2.18.4-py2.py3-none-any.whlCollectingidna=2.5(fromrequests)Usingcachedhttps://files.pythonhosted.org/packages/27/cc/6dd9a3869f15c2edfab863b992838277279ce92663d334df9ecf5106f5c6/idna-2.6-py2.py3-none-any.whlCollectingurllib3=1.21.1(fromrequests)Usingcachedhttps://files.pythonhosted.org/packages/63/cb/6965947c13a94236f6d4b8223e21beb4d576dc72e8130bd7880f600839b8/urllib3-1.22-py2.py3-none-any.whlCollectingcertifi>=2017.4.17(fromrequests)Usingcachedhttps://files.pythonhosted.org/packages/7c/e6/92ad559b7192d846975fc916b65f667c7b8c3a32bea7372340bfe9a15fa5/certifi-2018.4.16-py2.py3-none-any.whlCollectingchardet=3.0.2(fromrequests)Usingcachedhttps://files.pythonhosted.org/packages/bc/a9/01ffebfb562e4274b6487b4bb1ddec7ca55ec7510b22e4c51f14098443b8/chardet-3.0.4-py2.py3-none-any.whlInstallingcollectedpackages:idna,urllib3,certifi,chardet,requestsCouldnotinstallpackagesduetoanEnvironmentError:[Errno13]Permissiondenied:'/Library/Python/2.7/site-packages/idna'Considerusingthe`--user`optionorcheckthepermissions.我加了pipinstallrequest-user-U安装成功，但是在piplist查找不到，这是为啥呢？