Why won't pip install the current version of a package?

问题

I can't get pip to install the current version of pydot (1.0.28). Though yolk and PyPi both report this version as the available, current one,

pip -U pydot

gives me

Requirement already up-to-date: pydot in /Library/Python/2.7/site-packages
Requirement already up-to-date: pyparsing in /Library/Python/2.7/site-packages (from pydot)
Requirement already up-to-date: setuptools in /Library/Python/2.7/site-packages (from pydot)

forcing the current version with

pip -U pydot==1.0.28

gives me

Downloading/unpacking pydot==1.0.28
  Could not find a version that satisfies the requirement pydot==1.0.28 (from versions: 1.0.2)
  Some externally hosted files were ignored (use --allow-external to allow).
Cleaning up...
No distributions matching the version for pydot==1.0.28

and even

pip -U --allow-external pydot pydot==1.0.28

gives me

Downloading/unpacking pydot==1.0.28
  Could not find a version that satisfies the requirement pydot==1.0.28 (from versions: 1.0.2)
  Some insecure and unverifiable files were ignored (use --allow-unverified pydot to allow).
Cleaning up...
No distributions matching the version for pydot==1.0.28

回答1:

Try this:

pip install --allow-unverified pydot pydot==1.0.28

来源：https://stackoverflow.com/questions/22868777/why-wont-pip-install-the-current-version-of-a-package