在OS X上安装Python 3.3后出现“找不到命令”错误

Question

I was using Python 2.7 as it was installed on my Mac by default. I tried to update it to python 3.4. In the process I deleted 2.7 via the command line. I installed python 3.4 using an installer but I am still getting following error:

python: Command not found

I checked and found that this folder does not exist:

/Library/Frameworks/Python.framework/Versions/3.4

I followed the instructions on this website to update Python. Everything went well until step 5 but was unsuccessful for step 6 and 7.

I am in a complete mess as I have deleted the older version, Python 2.7. How can I fix my Python installation to solve the error and use Python 3.4?

Answer 1

While I agree with the several users who have commented that it's a bad idea to alter or remove the system's Python installation, it looks like the problem here is that you followed the steps given in the linked blog post to upgrade from version 2.x to a more recent version 2.x . Note where it says (emphasis mine):

To read about how to upgrade to Python 3.3, jump to the very bottom of this post.

Steps 1 through 7 in the blog post show how to upgrade the system Python from 2.7.1 to 2.7.3. While it would be fine to install them side-by-side instead—leaving you free to alter or delete your version without breaking the system version—you probably wouldn't run into trouble upgrading the system version from patch 1 to patch 3 of the same minor release.

However, Python 3 is not backwards-compatible with Python 2 . If your system version of Python is 2.x, replacing it with version 3.x just won't work; the system depends on having 2.x available. Note that the section at the bottom of the blog post titled "Updating to Python 3.3" does not remove Python 2.7:

rm -R /System/Library/Frameworks/Python.framework/Versions/3.3
mv /Library/Frameworks/Python.framework/Versions/3.3 /System/Library/Frameworks/Python.framework/Versions

What it does instead is remove any existing installation of the version to be upgraded , in this case Python 3.3, to avoid a Directory not empty error when trying to move the upgraded version into the system folder.

You need a working version of Python 2 in your system folder. If you want to use Python 3, you need a separate installation entirely. Rather than backtracking the steps of the guide in reverse order, you may be able to simply start again at step 1, but this time download and install Python 2 instead of Python 3, but you will need to make sure you understand what each step does, looking up shell commands if necessary, because it sounds like you've already made some modifications you don't understand in trying to adapt the wrong section to apply to Python 3.

For example, you might have changed the second command from step 2 from

sudo mv /Library/Frameworks/Python.framework/Versions/2.7 /System/Library/Frameworks/Python.framework/Versions

to

sudo mv /Library/Frameworks/Python.framework/Versions/3.4 /System/Library/Frameworks/Python.framework/Versions

when you were trying to use this step to "upgrade" from 2 to 3; otherwise, you would have gotten a No such file or directory error. If this is the case, you won't have a 2.7 folder to remove in the very first step because you already removed it the first time around and didn't replace it.