Coding and Decoding in Python

[Wanderer]

I have a dll that to communicate with I need to send numeric codes. So
I created a dictionary. It works in one direction in that I can
address the key and get the value. But when the program returns the
value I can't get the key. This code is very simple and I could use a
list and the index except for the last value. Is there a better way to
handle coding and decoding values to strings?

QCam_Info = {
'qinfCameraType' : 0, # Camera model (see
QCam_qcCameraType)
'qinfSerialNumber' : 1, # Deprecated
'qinfHardwareVersion' : 2, # Hardware version
'qinfFirmwareVersion' : 3, # Firmware version
'qinfCcd' : 4, # CCD model (see
QCam_qcCcd)
'qinfBitDepth' : 5, # Maximum bit depth
'qinfCooled' : 6, # Returns 1 if
cooler is available, 0 if not
'qinfReserved1' : 7, # Reserved
'qinfImageWidth ' : 8, # Width of the ROI (in
pixels)
'qinfImageHeight' : 9, # Height of the ROI (in
pixels)
'qinfImageSize' : 10, # Size of returned
image (in bytes)
'qinfCcdType' : 11, # CDD type (see
QCam_qcCcdType)
'qinfCcdWidth' : 12, # CCD maximum width
'qinfCcdHeight' : 13, # CCD maximum height
'qinfFirmwareBuild' : 14, # Build number of the
firmware
'qinfUniqueId' : 15, # Same as uniqueId
in QCam_CamListItem
'qinfIsModelB' : 16, # Cameras
manufactured after March 1, 2004 return 1, otherwise 0
'qinfIntensifierModel' : 17, # Intensifier tube
model (see QCam_qcIntensifierModel)
'qinfExposureRes' : 18, # Exposure time
resolution (nanoseconds)
'qinfTriggerDelayRes' : 19, # Trigger delay
Resolution (nanoseconds)
'qinfStreamVersion' : 20, # Streaming version
'qinfNormGainSigFigs' : 21, # Normalized Gain
Significant Figures resolution
'qinfNormGaindBRes' : 22, # Normalized Gain dB
resolution (in micro units)
'qinfNormITGainSigFigs' : 23, # Normalized Intensifier
Gain Significant Figures
'qinfNormITGaindBRes' : 24, # Normalized Intensifier
Gain dB resolution (micro units)
'qinfRegulatedCooling' : 25, # 1 if camera has
regulated cooling
'qinfRegulatedCoolingLock' : 26, # 1 if camera is at
regulated temperature, 0 otherwise
'qinfFanControl' : 29, # 1 if camera can
control fan speed
'qinfHighSensitivityMode' : 30, # 1 if camera has high
sensitivity mode available
'qinfBlackoutMode' : 31, # 1 if camera has
blackout mode available
'qinfPostProcessImageSize' : 32, # Returns the size (in
bytes) of the post-processed image
'qinfAsymmetricalBinning' : 33, # 1 if camera has
asymmetrical binning (ex: 2x4)
'qinfEMGain' : 34, # 1 if EM gain is
supported, 0 if not
'qinfOpenDelay' : 35, # 1 if shutter open
delay controls are available, 0 if not
'qinfCloseDelay' : 36, # 1 if shutter close
delay controls are available, 0 if not
'qinfColorWheelSupported' : 37, # 1 if color wheel is
supported, 0 if not
'qinfReserved2' : 38,
'qinfReserved3' : 39,
'qinfReserved4' : 40,
'qinfReserved5' : 41,
'qinfEasyEmModeSupported' : 42, # 1 if camera supports
Easy EM mode
'qinfLockedGainModeSupported' : 43,
'qinf_last' : 44,
'_qinf_force32' : 0xFFFFFFFF
}

 

[Mel]

I have a dll that to communicate with I need to send numeric codes. So
I created a dictionary. It works in one direction in that I can
address the key and get the value. But when the program returns the
value I can't get the key. This code is very simple and I could use a
list and the index except for the last value. Is there a better way to
handle coding and decoding values to strings?

QCam_Info = {
'qinfCameraType' : 0, # Camera model (see
QCam_qcCameraType)
'qinfSerialNumber' : 1, # Deprecated
'qinfHardwareVersion' : 2, # Hardware version
'qinfFirmwareVersion' : 3, # Firmware version
'qinfCcd' : 4, # CCD model (see
QCam_qcCcd)

[ ... ]
'_qinf_force32' : 0xFFFFFFFF

}

I handled this problem in a kind of cheap, nasty way with (untested)

for k, v in QCam_Info.items():
QCam_Info[v] = k

Then the dictionary lookups work both ways.

Mel.

 

[John Gordon]

But when the program returns the value I can't get the key.

What happens when two keys have the same value? How would you know which
key to return?

In your sample code all the values are different, but surely that won't
always be the case with real data.

 

[Wanderer]

What happens when two keys have the same value?  How would you know which
key to return?

In your sample code all the values are different, but surely that won't
always be the case with real data.

I guess two keys having the same value is why dictionaries don't
return keys for values, but this is a code. Each value has a unique
meaning to both sender and receiver. The text part is for making the
program understandable and printing understandable error messages.

 

[John Gordon]

I guess two keys having the same value is why dictionaries don't
return keys for values, but this is a code. Each value has a unique
meaning to both sender and receiver. The text part is for making the
program understandable and printing understandable error messages.

I see. You're storing integer equivalents for the labels themselves,
not the actual data associated with the labels.

Then Mel's solution is a good one -- construct a second dict which
has the keys and values swapped.

 

[Steven D'Aprano]

I have a dll that to communicate with I need to send numeric codes. So I
created a dictionary. It works in one direction in that I can address
the key and get the value. But when the program returns the value I
can't get the key.

If you only have a few keys:

def find_key(value, d):
"""Return the key in dictionary d that has the given value.
If there are multiple keys with the same value, return an
arbitrarily chosen one."""
for k, v in d.items():
if v == value: return k
raise KeyError('no such value found')


If you have many keys/values, then simply create a reverse dictionary:

Rev_QCam_Info = {}
for key, value in QCam_Info.items():
Rev_QCam_Info[value] = key

and then search that.