Installing Py on Windows: Need to restart or logout for path to be added

[JohnPalermo]

BPO	24757
Nosy	@pfmoore, @tjguk, @zware, @eryksun, @zooba

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2020-09-24.18:03:48.594>
created_at = <Date 2015-07-30.17:55:15.532>
labels = ['3.8', 'type-feature', 'expert-installation', 'OS-windows']
title = 'Installing Py on Windows: Need to restart or logout for path to be added'
updated_at = <Date 2020-09-24.23:20:17.896>
user = 'https://bugs.python.org/JohnPalermo'

bugs.python.org fields:

activity = <Date 2020-09-24.23:20:17.896>
actor = 'eryksun'
assignee = 'docs@python'
closed = True
closed_date = <Date 2020-09-24.18:03:48.594>
closer = 'steve.dower'
components = ['Installation', 'Windows']
creation = <Date 2015-07-30.17:55:15.532>
creator = 'John Palermo'
dependencies = []
files = []
hgrepos = []
issue_num = 24757
keywords = []
message_count = 6.0
messages = ['247688', '247691', '247757', '377449', '377453', '377468']
nosy_count = 8.0
nosy_names = ['paul.moore', 'tim.golden', 'docs@python', 'zach.ware', 'eryksun', 'steve.dower', 'John Palermo', 'forrestshields2']
pr_nums = []
priority = 'normal'
resolution = 'out of date'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue24757'
versions = ['Python 2.7', 'Python 3.4', 'Python 3.8']

[JohnPalermo]

Something I suppose many new users could stumble over: After installing Python and trying out "pip" or "python" on the command line nothing is found. You have to re-start Windows or re-log into your account.

I suggest adding this information to the documentation here

https://docs.python.org/2/using/windows.html#installing-python

and also adding it to the Windows installer description for the "add Python to your path" option.

[eryksun]

I think a custom action can be added to Tools/msi/msisupport.c to send a WM_SETTINGCHANGE "Environment" message to top-level windows. This makes Explorer reload its environment from the registry, so starting a new command prompt (cmd.exe) from Explorer will see the updated PATH.

[zooba]

That's exactly what is needed (though it still won't affect command prompts that are already open).

The 3.5+ installer does it, so this only affects 2.7 and 3.4.

[forrestshields2]

As of 3.8.5 the current shell's PATH is outdated after a commandline system-wide installation. This makes it very difficult to do a scripted install of Python followed by the python or pip commands, as they will not be found. In addition to modifying the PATH in the system scope, the PATH should also be modified for the current process (shell).

Here is the WORKAROUND I created and am currently using:

1. Perform the commandline system-wide installation from an Administrative PowerShell prompt.
2. After the installation (but before using python or pip) rebuild the PATH environment variable for the current process from the concatenation of the PATH environment variables from the System and User scopes (this is how Windows builds the PATH) by using this code:

[Environment]::SetEnvironmentVariable('PATH', [Environment]::GetEnvironmentVariable('PATH', 'Machine') + ';' + [Environment]::GetEnvironmentVariable('PATH', 'User'), 'Process')

[zooba]

Sorry, but we don't have any way to update the current terminal process. We already update the current shell, so if you start a new terminal/Powershell/cmd/etc. instance it should have the updated variable.

The workaround you posted is fine. Directly adding the new install to the current PATH is also okay.

[eryksun]

We already update the current shell, so if you start a new
terminal/Powershell/cmd/etc. instance it should have the
updated variable.

The installer broadcasts an "Environment" message to top-level windows. In practice, only Explorer listens for and reloads its environment for this window message. For most programs, a new instance has to be run from Explorer in order to get the updated environment.

CMD and PowerShell don't own any windows, so they cannot get this message in principle. The console-session host process (conhost or openconsole) may own a top-level window if it wasn't allocated with CREATE_NO_WINDOW and wasn't created as a ConPTY server (e.g. under Windows Terminal). But the console doesn't have a console control event for this case (e.g. something like a hypothetical CTRL_ENVIRONMENT_EVENT) that it sends to client processes. It's assumed that console applications that need to interact with the desktop environment will create at least a hidden window.

Without the "Environment" message, it's still possible for the user to run a script that reloads the environment in place in CMD or PowerShell. The environment has to be loaded in four passes: system REG_SZ values, system REG_EXPAND_SZ values, user REG_SZ values, and user REG_EXPAND_SZ values. User values should always be able to depend on system values, and REG_EXPAND_SZ values should always be able to depend on REG_SZ values. This is the way WinAPI CreateEnvironmentBlock has always worked. When reloading PATH, this is an issue if a new machine PATH value depends on new/modified REG_SZ system variables or if a new user PATH value depends on new/modified system variables or new/modified REG_SZ user variables.