According to the docs, Inno Setup uses AppName or AppId to allow you to create an update program that will automatically put its files in the same path which the user installed the initial application to.
I need to be able to determine where Inno Setup installed files to based on AppId, but NOT from within Inno Setup. For example, I need to be able to determine this from a Python script.
One use case: patching a file in the installed program location. It would be overkill to package an entire installer just to, say, conditionally add or edit a line in a text file. A simple Python script could accomplish this, plus the user could review the script if they desired. I cannot and should not assume the user just installed to the default location, hence why I need to be able to see where the user installed the program.
Inno Setup obviously stores this somewhere since it is able to make its own patches, but I can't seem to find it in the registry. I've searched the Registry for my app ID but I only see it in the Uninstall section. I can probably pull it from there, but I think you can also create installers without uninstallers – where would this end up in that case?
The path is stored to registry to HKLM (administrative install mode) or HKCU (non administrative install mode) to a subkey named after the AppId with _is1 suffix, stored under a key SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall (as you alraedy know). The value name is Inno Setup: App Path. The path is also stored to InstallLocation with additional trailing slash, as that's where Windows reads it from. But Inno Setup reads the first value.
An example for HKLM:
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall\appid_is1]
"Inno Setup: App Path"="C:\\Program Files\\My Program"
"InstallLocation"="C:\\Program Files\\My Program\\"
You can see this in dozens questions, that show how to read the value in Inno Setup code. For example:
How to get path of installation of target game/application from registry when installing mod/plugin using Inno Setup?
If you create an installer what cannot be uninstalled (CreateUninstallRegKey=no or CreateUninstallRegKey=no), then the path is not store anywhere. In such case you would have to explicitly code your installer to store the path for you somewhere.