How can I retrieve the signal strength of nearby wireless LAN networks on Windows using Python?

I would like to either show or graph the values.

Solution

If you are on Windows, you probably want to use the WLAN API, which provides the 'WlanGetAvailableNetworkList()' function (see the API docs for usage). I am not aware of any python wrappers for WLANAPI.DLL so you may have to wrap it yourself using ctypes. I have a preliminary script that does this (works-for-me), but it may be crufty. You'll want to read the documentation to understand the meaning of all the fields:

from ctypes import *
from ctypes.wintypes import *
from sys import exit


def customresize(array, new_size):
    return (array._type_*new_size).from_address(addressof(array))

wlanapi = windll.LoadLibrary('wlanapi.dll')

ERROR_SUCCESS = 0

class GUID(Structure):
    _fields_ = [
        ('Data1', c_ulong),
        ('Data2', c_ushort),
        ('Data3', c_ushort),
        ('Data4', c_ubyte*8),
        ]

WLAN_INTERFACE_STATE = c_uint
(wlan_interface_state_not_ready,
 wlan_interface_state_connected,
 wlan_interface_state_ad_hoc_network_formed,
 wlan_interface_state_disconnecting,
 wlan_interface_state_disconnected,
 wlan_interface_state_associating,
 wlan_interface_state_discovering,
 wlan_interface_state_authenticating) = map(WLAN_INTERFACE_STATE, range(0, 8))

class WLAN_INTERFACE_INFO(Structure):
    _fields_ = [
        ("InterfaceGuid", GUID),
        ("strInterfaceDescription", c_wchar * 256),
        ("isState", WLAN_INTERFACE_STATE)
        ]

class WLAN_INTERFACE_INFO_LIST(Structure):
    _fields_ = [
        ("NumberOfItems", DWORD),
        ("Index", DWORD),
        ("InterfaceInfo", WLAN_INTERFACE_INFO * 1)
        ]

WLAN_MAX_PHY_TYPE_NUMBER = 0x8
DOT11_SSID_MAX_LENGTH = 32
WLAN_REASON_CODE = DWORD
DOT11_BSS_TYPE = c_uint
(dot11_BSS_type_infrastructure,
 dot11_BSS_type_independent,
 dot11_BSS_type_any) = map(DOT11_BSS_TYPE, range(1, 4))
DOT11_PHY_TYPE = c_uint
dot11_phy_type_unknown      = 0
dot11_phy_type_any          = 0
dot11_phy_type_fhss         = 1
dot11_phy_type_dsss         = 2
dot11_phy_type_irbaseband   = 3
dot11_phy_type_ofdm         = 4
dot11_phy_type_hrdsss       = 5
dot11_phy_type_erp          = 6
dot11_phy_type_ht           = 7
dot11_phy_type_IHV_start    = 0x80000000
dot11_phy_type_IHV_end      = 0xffffffff 

DOT11_AUTH_ALGORITHM = c_uint
DOT11_AUTH_ALGO_80211_OPEN         = 1
DOT11_AUTH_ALGO_80211_SHARED_KEY   = 2
DOT11_AUTH_ALGO_WPA                = 3
DOT11_AUTH_ALGO_WPA_PSK            = 4
DOT11_AUTH_ALGO_WPA_NONE           = 5
DOT11_AUTH_ALGO_RSNA               = 6
DOT11_AUTH_ALGO_RSNA_PSK           = 7
DOT11_AUTH_ALGO_IHV_START          = 0x80000000
DOT11_AUTH_ALGO_IHV_END            = 0xffffffff

DOT11_CIPHER_ALGORITHM = c_uint
DOT11_CIPHER_ALGO_NONE            = 0x00
DOT11_CIPHER_ALGO_WEP40           = 0x01
DOT11_CIPHER_ALGO_TKIP            = 0x02
DOT11_CIPHER_ALGO_CCMP            = 0x04
DOT11_CIPHER_ALGO_WEP104          = 0x05
DOT11_CIPHER_ALGO_WPA_USE_GROUP   = 0x100
DOT11_CIPHER_ALGO_RSN_USE_GROUP   = 0x100
DOT11_CIPHER_ALGO_WEP             = 0x101
DOT11_CIPHER_ALGO_IHV_START       = 0x80000000
DOT11_CIPHER_ALGO_IHV_END         = 0xffffffff 

WLAN_AVAILABLE_NETWORK_CONNECTED = 1
WLAN_AVAILABLE_NETWORK_HAS_PROFILE = 2

WLAN_AVAILABLE_NETWORK_INCLUDE_ALL_ADHOC_PROFILES = 0x00000001
WLAN_AVAILABLE_NETWORK_INCLUDE_ALL_MANUAL_HIDDEN_PROFILES = 0x00000002

class DOT11_SSID(Structure):
    _fields_ = [
        ("SSIDLength", c_ulong),
        ("SSID", c_char * DOT11_SSID_MAX_LENGTH)
        ]

class WLAN_AVAILABLE_NETWORK(Structure):
    _fields_ = [
        ("ProfileName", c_wchar * 256),
        ("dot11Ssid", DOT11_SSID),
        ("dot11BssType", DOT11_BSS_TYPE),
        ("NumberOfBssids", c_ulong),
        ("NetworkConnectable", c_bool),
        ("wlanNotConnectableReason", WLAN_REASON_CODE),
        ("NumberOfPhyTypes", c_ulong),
        ("dot11PhyTypes", DOT11_PHY_TYPE * WLAN_MAX_PHY_TYPE_NUMBER),
        ("MorePhyTypes", c_bool),
        ("wlanSignalQuality", c_ulong),
        ("SecurityEnabled", c_bool),
        ("dot11DefaultAuthAlgorithm", DOT11_AUTH_ALGORITHM),
        ("dot11DefaultCipherAlgorithm", DOT11_CIPHER_ALGORITHM),
        ("Flags", DWORD),
        ("Reserved", DWORD)
        ]

class WLAN_AVAILABLE_NETWORK_LIST(Structure):
    _fields_ = [
        ("NumberOfItems", DWORD),
        ("Index", DWORD),
        ("Network", WLAN_AVAILABLE_NETWORK * 1)
        ]

WlanOpenHandle = wlanapi.WlanOpenHandle
WlanOpenHandle.argtypes = (DWORD, c_void_p, POINTER(DWORD), POINTER(HANDLE))
WlanOpenHandle.restype = DWORD

WlanEnumInterfaces = wlanapi.WlanEnumInterfaces
WlanEnumInterfaces.argtypes = (HANDLE, c_void_p, 
                               POINTER(POINTER(WLAN_INTERFACE_INFO_LIST)))
WlanEnumInterfaces.restype = DWORD

WlanGetAvailableNetworkList = wlanapi.WlanGetAvailableNetworkList
WlanGetAvailableNetworkList.argtypes = (HANDLE, POINTER(GUID), DWORD, c_void_p, 
                                        POINTER(POINTER(WLAN_AVAILABLE_NETWORK_LIST)))
WlanGetAvailableNetworkList.restype = DWORD

WlanFreeMemory = wlanapi.WlanFreeMemory
WlanFreeMemory.argtypes = [c_void_p]


if __name__ == '__main__':
    NegotiatedVersion = DWORD()
    ClientHandle = HANDLE()
    ret = WlanOpenHandle(1, None, byref(NegotiatedVersion), byref(ClientHandle))
    if ret != ERROR_SUCCESS:
        exit(FormatError(ret))

    # find all wireless network interfaces
    pInterfaceList = pointer(WLAN_INTERFACE_INFO_LIST())
    ret = WlanEnumInterfaces(ClientHandle, None, byref(pInterfaceList))
    if ret != ERROR_SUCCESS:
        exit(FormatError(ret))

    try:
        ifaces = customresize(pInterfaceList.contents.InterfaceInfo,
                              pInterfaceList.contents.NumberOfItems)
        # find each available network for each interface
        for iface in ifaces:
            print("Interface: {}".format(iface.strInterfaceDescription))
            pAvailableNetworkList = pointer(WLAN_AVAILABLE_NETWORK_LIST())
            ret = WlanGetAvailableNetworkList(ClientHandle, 
                                        byref(iface.InterfaceGuid),
                                        0,
                                        None,
                                        byref(pAvailableNetworkList))
            if ret != ERROR_SUCCESS:
                exit(FormatError(ret))
            try:
                avail_net_list = pAvailableNetworkList.contents
                networks = customresize(avail_net_list.Network, 
                                        avail_net_list.NumberOfItems)
                for network in networks:
                    print("SSID: {}, quality: {:2d}%".format(
                        network.dot11Ssid.SSID[:network.dot11Ssid.SSIDLength].decode(),
                        network.wlanSignalQuality))
            finally:
                WlanFreeMemory(pAvailableNetworkList)
    finally:
        WlanFreeMemory(pInterfaceList)

On linux, you have a couple of choices:

The standard options is to parse the output of iwlist -scan. However, if you are currently connected to a wlan and not running as root, it only returns the currently connected wlan.

If you need more than this, the best way is to query the wireless manager daemon. On modern, linuxes, this is usually NetworkManager, although wicd is becoming more popular. Both of these managers can be queried using dbus. Unless you can control what the clients have on their systems, you might need to support both of these options, or at least one option and have a fallback for iwlist.