I'm trying to use Python in PowerShell

For what's worth, this command did it for me (Python 3.3):

[System.Environment]::SetEnvironmentVariable("PATH", $Env:Path + ";C:\Python33", "Machine")

I just had to restart the PowerShell window after that.

$env:path="$env:Path;C:\Python27" will only set it for the current session. Next time you open PowerShell, you will have to do the same thing again.

The [Environment]::SetEnvironmentVariable() is the right way, and it would have set your PATH environment variable permanently. You just have to start PowerShell again to see the effect in this case.

The directory is not set correctly, so please follow these steps.

1. "MyComputer" → Right Click → Properties → "System Properties" → "Advanced" tab

2. "Environment Variables" → "Path" → "Edit"

3. In the "Variable value" box, make sure you see following:

   ;c:\python27\;c:\python27\scripts
   

4. Click OK. Test this change by restarting your Windows PowerShell. Type

   python
   

5. Now Python version 2 runs! Yay!

For a permanent solution I found the following worked:

[Environment]::SetEnvironmentVariable("Path", "$env:Path;C:\Python 3.5")

For versions above Python 3.3, using the python command directly is no longer optimal and may lead to unexpected things like opening of Windows Store.
It's now common to have multiple python versions in different folders which are installed by different programs and users, and they may conflict if there is a default one persistently available from the command line.

The intended way is to use some helper tool to dynamically select one of the folders containing python*.exe (these folders can also be referred as environments or versions). For example, Anaconda uses simple bat file for this.
The default tool for Windows is python launcher, which is included in the official installation (and which is probably already installed on your system). Try simple py command where previously python was used. More details here.

For example, to check if launcher is presented and which installed python versions it finds: py --list-paths To run script: py -3. main.py or simply py main.py
To use directly in PowerShell as asked in this question: py

Additional confirmation.

This works for me permanently:

[Environment]::SetEnvironmentVariable("Path", "$env:Path;C:\Python27","User")

From the Python Guide, this is what worked for me (Python 2.7.9): [Environment]::SetEnvironmentVariable("Path", "$env:Path;C:\Python27\;C:\Python27\Scripts\", "User")

Sometimes you install Python on Windows and it doesn't configure the path correctly.

Make sure you enter [Environment]::SetEnvironmentVariable("Path", "$env:Path;C:\Python27", "User")
in PowerShell to configure it correctly.

You also have to either restart PowerShell or your whole computer to get it to really be fixed.

As MaxPRafferty mentioned:

Just a note to anyone landing here from google, the answers setting path are all correct, but this problem probably stems from not giving the python installer administrative rights it needs to set the path itself. An alternative may be to simply right click the installer and select run as administrator, then repair the installation. If that still doesn't work, choose the [Environment] answer below that corresponds to your python version and installation directory. – MaxPRafferty Nov 18 '15 at 20:06

Maybe it is wise to let Python installer to add the path itself. The trap here is that by default Python installer does NOT add path for you. You should look carefully (by scrolling down to see what has been installed) during the installation process instead of directly nexting to the end.

What he missed saying is that you cannot run as administrator once you have installed it. Uninstall and reinstall may do, but the simplest is to right click and Troubleshoot compatibility, being careful this time to check the 'add path' in the "what to install" dialog before hiting next. Then restart powershell. Voilà. It works for me.

To be able to use Python immediately without restarting the shell window you need to change the path for the machine, the process and the user.

Function Get-EnvVariableNameList {
    [cmdletbinding()]
    $allEnvVars = Get-ChildItem Env:
    $allEnvNamesArray = $allEnvVars.Name
    $pathEnvNamesList = New-Object System.Collections.ArrayList
    $pathEnvNamesList.AddRange($allEnvNamesArray)
    return ,$pathEnvNamesList
}

Function Add-EnvVarIfNotPresent {
Param (
[string]$variableNameToAdd,
[string]$variableValueToAdd
   ) 
    $nameList = Get-EnvVariableNameList
    $alreadyPresentCount = ($nameList | Where{$_ -like $variableNameToAdd}).Count
    #$message = ''
    if ($alreadyPresentCount -eq 0)
    {
    [System.Environment]::SetEnvironmentVariable($variableNameToAdd, $variableValueToAdd, [System.EnvironmentVariableTarget]::Machine)
    [System.Environment]::SetEnvironmentVariable($variableNameToAdd, $variableValueToAdd, [System.EnvironmentVariableTarget]::Process)
    [System.Environment]::SetEnvironmentVariable($variableNameToAdd, $variableValueToAdd, [System.EnvironmentVariableTarget]::User)
        $message = "Enviromental variable added to machine, process and user to include $variableNameToAdd"
    }
    else
    {
        $message = 'Environmental variable already exists. Consider using a different function to modify it'
    }
    Write-Information $message
}


Function Get-EnvExtensionList {
    $pathExtArray =  ($env:PATHEXT).Split("{;}")
    $pathExtList = New-Object System.Collections.ArrayList
    $pathExtList.AddRange($pathExtArray)
    return ,$pathExtList
}


Function Add-EnvExtension {
Param (
[string]$pathExtToAdd
   ) 
    $pathList = Get-EnvExtensionList
    $alreadyPresentCount = ($pathList | Where{$_ -like $pathToAdd}).Count
    if ($alreadyPresentCount -eq 0)
    {
        $pathList.Add($pathExtToAdd)
        $returnPath = $pathList -join ";"
        [System.Environment]::SetEnvironmentVariable('pathext', $returnPath, [System.EnvironmentVariableTarget]::Machine)
        [System.Environment]::SetEnvironmentVariable('pathext', $returnPath, [System.EnvironmentVariableTarget]::Process)
        [System.Environment]::SetEnvironmentVariable('pathext', $returnPath, [System.EnvironmentVariableTarget]::User)
        $message = "Path extension added to machine, process and user paths to include $pathExtToAdd"
    }
    else
    {
        $message = 'Path extension already exists'
    }
    Write-Information $message
}

Function Get-EnvPathList {
    [cmdletbinding()]
    $pathArray =  ($env:PATH).Split("{;}")
    $pathList = New-Object System.Collections.ArrayList
    $pathList.AddRange($pathArray)
    return ,$pathList
}

Function Add-EnvPath {
Param (
[string]$pathToAdd
   ) 
    $pathList = Get-EnvPathList
    $alreadyPresentCount = ($pathList | Where{$_ -like $pathToAdd}).Count
    if ($alreadyPresentCount -eq 0)
    {
        $pathList.Add($pathToAdd)
        $returnPath = $pathList -join ";"
        [System.Environment]::SetEnvironmentVariable('path', $returnPath, [System.EnvironmentVariableTarget]::Machine)
        [System.Environment]::SetEnvironmentVariable('path', $returnPath, [System.EnvironmentVariableTarget]::Process)
        [System.Environment]::SetEnvironmentVariable('path', $returnPath, [System.EnvironmentVariableTarget]::User)
        $message = "Path added to machine, process and user paths to include $pathToAdd"
    }
    else
    {
        $message = 'Path already exists'
    }
    Write-Information $message
}

Add-EnvExtension '.PY'
Add-EnvExtension '.PYW'
Add-EnvPath 'C:\Python27\'

Try the command this way:

cd C:\Users\XXX
python.exe

instead of:

cd C:\Users\XXX
python

Just eliminate the word "User". It will work.