You get the AttributeError: __enter__ error because xl.workbooks.open is not a context manager, and so it doesn't support the with statement.

If you want to use a with statement in your code you can use the closing function from the contextlib module in the standard library, like this:

from contextlib import closing

def wb_ref(file):
    xl.DisplayAlerts = False
    with closing(xl.workbooks.open(file)) as wb:
        wb.RefreshAll()
        wb.Save()

contextlib.closing will automatically call close on the object that is passed to it when the code in the with block has completed execution.

For those who find the accepted answer not working (look here for the reasons) - there's still a way to get objects having a _prop_map_get_ attribute (a dict that holds object's fields as keys). You just have to create the main app object with win32com.client.gencache.EnsureDispatch().

Here's a convenience function I wrote that lists fields and methods of a passed COM object created that way:

from inspect import getmembers


def print_members(obj, obj_name="placeholder_name"):
    """Print members of given COM object"""
    try:
        fields = list(obj._prop_map_get_.keys())
    except AttributeError:
        print("Object has no attribute '_prop_map_get_'")
        print("Check if the initial COM object was created with"
              "'win32com.client.gencache.EnsureDispatch()'")
        raise
    methods = [m[0] for m in getmembers(obj) if (not m[0].startswith("_")
                                                 and "clsid" not in m[0].lower())]

    if len(fields) + len(methods) > 0:
        print("Members of '{}' ({}):".format(obj_name, obj))
    else:
        raise ValueError("Object has no members to print")

    print("\tFields:")
    if fields:
        for field in fields:
            print(f"\t\t{field}")
    else:
        print("\t\tObject has no fields to print")

    print("\tMethods:")
    if methods:
        for method in methods:
            print(f"\t\t{method}")
    else:
        print("\t\tObject has no methods to print")

For an Excel object created with win32com.client.gencache.EnsureDispatch("Excel.Application") its output would be:

Members of 'Excel.Application' (Microsoft Excel):
    Fields:
        ActiveCell
        ActiveChart
        ActiveDialog
        ActiveEncryptionSession
        ...
        Workbooks
        WorksheetFunction
        Worksheets
        _Default
    Methods:
        ActivateMicrosoftApp
        AddChartAutoFormat
        AddCustomList
        Calculate
        ...
        Union
        Volatile
        Wait

Some citations:

As we discussed previously, automation objects are COM objects that expose methods and properties using the IDispatch interface. So how do we use these objects from Python? The win32com.client package contains a number of modules to provide access to automation objects. This package supports both late and early bindings, as we will discuss.

To use an IDispatch-based COM object, use the method win32com.client.Dispatch(). This method takes as its first parameter the ProgID or CLSID of the object you wish to create. If you read the documentation for Microsoft Excel, you'll find the ProgID for Excel is Excel.Application, so to create an object that interfaces to Excel, use the following code:

import win32com.client
xl = win32com.client.Dispatch("Excel.Application")

(from this)

The WScript.Shell object provides functions to read system information and environment variables, work with the registry and manage shortcuts. (from: 1 2)