Weird behavior in Python in array initialization

Question

In the following you see a snippet of my Python program that I wrote to send APDU commands to smart cards. I used PySCard library in this program.

First Program :

for LOAD_KEY in validLoadCommands:
 data,sw1,sw2 =connection.transmit(LOAD_KEY)
 temp = LOAD_KEY
 x= temp[3]
 for j in range(0,len(LOAD_KEY)):
 LOAD_KEY[j]=hex(LOAD_KEY[j])
 print ' >>>',' Load Command :',
 for j in range(0,len(LOAD_KEY)):
 LOAD_KEY[j]=str(LOAD_KEY[j])
 if len(LOAD_KEY[j])==3:
 LOAD_KEY[j]=LOAD_KEY[j][0:2]+'0'+LOAD_KEY[j][2]
 print LOAD_KEY[j],
 print
 for j in range(0,len(data)):
 data[j]=hex(data[j])
 print ' <<<',' Data :',
 for j in range(0,len(data)):
 data[j]=str(data[j])
 if len(data[j])==3:
 data[j]=data[j][0:2]+'0'+data[j][2]
 print data[j],
 print '--',hex(sw1),hex(sw2)
 if (temp[2] == 0x00 or temp[2] == 0x20):
 keyType = 0x60
 else:
 keyType = 0x61
 AUTH = [0xFF, 0x86, 0x00, 0x00, 0x05, 0x01, 0x00, blockNum, keyType,temp[3]]
 print 'AUTH Command: ', AUTH
 data,sw1,sw2 =connection.transmit(AUTH)
 print data, sw1, sw2

And its output :

Auth Command: [255, 134, 0, 0, 5, 1, 0, 4, 97, '0x0e']
Traceback (most recent call last):
 File "C:\Users\Erb4h1m\Desktop\Faraadis Alborz Card Reader\CRT-603-CZ1_Read1Block.py", line 199, in <module>
 data,sw1,sw2 =connection.transmit(AUTH)
 File "D:\Software\Python27\lib\site-packages\smartcard\CardConnectionDecorator.py", line 82, in transmit
 return self.component.transmit(bytes, protocol)
 File "D:\Software\Python27\lib\site-packages\smartcard\CardConnection.py", line 140, in transmit
 data, sw1, sw2 = self.doTransmit(bytes, protocol)
 File "D:\Software\Python27\lib\site-packages\smartcard\pcsc\PCSCCardConnection.py", line 173, in doTransmit
 hresult, response = SCardTransmit(self.hcard, pcscprotocolheader, bytes)
 File "D:\Software\Python27\lib\site-packages\smartcard\scard\scard.py", line 1329, in SCardTransmit
 return _scard.SCardTransmit(*args)
TypeError: Expected a list of bytes.

And when I replace temp[3] with x:

for LOAD_KEY in validLoadCommands:
 data,sw1,sw2 =connection.transmit(LOAD_KEY)
 temp = LOAD_KEY
 x= temp[3]
 for j in range(0,len(LOAD_KEY)):
 LOAD_KEY[j]=hex(LOAD_KEY[j])
 print ' >>>',' Load Command :',
 for j in range(0,len(LOAD_KEY)):
 LOAD_KEY[j]=str(LOAD_KEY[j])
 if len(LOAD_KEY[j])==3:
 LOAD_KEY[j]=LOAD_KEY[j][0:2]+'0'+LOAD_KEY[j][2]
 print LOAD_KEY[j],
 print
 for j in range(0,len(data)):
 data[j]=hex(data[j])
 print ' <<<',' Data :',
 for j in range(0,len(data)):
 data[j]=str(data[j])
 if len(data[j])==3:
 data[j]=data[j][0:2]+'0'+data[j][2]
 print data[j],
 print '--',hex(sw1),hex(sw2)
 if (temp[2] == 0x00 or temp[2] == 0x20):
 keyType = 0x60
 else:
 keyType = 0x61
 AUTH = [0xFF, 0x86, 0x00, 0x00, 0x05, 0x01, 0x00, blockNum, keyType, x]
 print 'AUTH Command: ', AUTH
 data,sw1,sw2 =connection.transmit(AUTH)
 print data, sw1, sw2

Then the output is:

AUTH: [255, 134, 0, 0, 5, 1, 0, 4, 97, 14]
[] 144 0

As you see above, in the second case I DON'T receive any error, while there is no difference between first program and second program! I only replaced

 AUTH = [0xFF, 0x86, 0x00, 0x00, 0x05, 0x01, 0x00, blockNum, keyType,temp[3]]

with:

 AUTH = [0xFF, 0x86, 0x00, 0x00, 0x05, 0x01, 0x00, blockNum, keyType,x]

and x is initialized with temp[3] in the top of program. neither x nor temp[3] didn't changed between the line of initialization and the line of error.

What is going here?

• Dunes: Why are you converting your lists of ints into lists of strings? All it seems to be doing is causing you a headache. For instance, you if statement will always fail as you testing for equality between a string and an int.
• Dunes: If you really want to create strings for printing then only convert them when you need to print them (and don't modify the list). "0x{:0>2x}".format(10) produces "0x0a". The bit between the braces is the format specifier for your number. The colon denotes talking about how to format the argument. 0>2 align the number on the right, padding up to at least two characters (if needed) with zeros. The x means to format the number in hexadecimal.
• Ebrahim Ghasemi: @Dunes When I change LOAD_KEY in the program, does the value of temp change also?
• Dunes: Yes. Because LOAD_KEY and temp are the same list object in memory. Perhaps you want temp to be a copy of LOAD_KEY. Use temp = list(LOAD_KEY) or temp = LOAD_KEY[:] to create a copy.
• Ebrahim Ghasemi: @Dunes Your comments must written in gold! :) Those are really useful. Thank you.

Answer 1

Your temp variable reference LOAD_KEY (Line 3: temp = LOAD_KEY)

When x= temp[3] get called, it's the value of LOAD_KEY at index 3 right after your connection.transmit that is set.

When you are sending temp[3] for your AUTH, LOAD_KEY have changed and so does temp[3]. So it's not anymore the same value as x.

connection.transmit => LOAD_KEY is set => temp => [3] => x
*** LOAD_KEY changes ***
x => old value of temp[3] aka LOAD_KEY[3]
temp[3] => whatever your code have put in it

Edit: short example

>>> LOAD_KEY = ['one', 'two', 'three', 'four']
>>> temp = LOAD_KEY
>>> x = temp[3]
>>> LOAD_KEY[3] = 'new value'
>>>
>>> LOAD_KEY[3]
'new value'
>>> temp[3]
'new value'
>>> x
'four'

• Ebrahim Ghasemi: Your temp variable reference LOAD_KEY (Line 3: temp = LOAD_KEY) So when I change LOAD_KEY, the value of temp also change?
• Ebrahim Ghasemi: In the line x= temp[3] the x variable reference temp[3]. So when I change LOAD_KEY the value of temp will change automatically, and when temp change, the value of temp[3] and x variable will change automatically, Am I right?
• Cyrbil: See my edit for a comprehensible (hopefully) example.

Answer 2

In you first example, temp[3] is a string ('0x0e').

In your second example, x is an integer (14).

This is because you're converting the integer values of the LOAD_KEY list to an hexadecimal string with the following line:

LOAD_KEY[j] = hex(LOAD_KEY[j])

In order to convert a string in hexadecimal representation to an integer, use the second argument of the built-in type int to set the base to 16:

>>> int('0x0e', 16)
14

• Cyrbil: You are right too, probably a mistake at line 9 LOAD_KEY[j]=str(LOAD_KEY[j])
• Dunes: The conversion to a string is earlier than that -- LOAD_KEY[j]=hex(LOAD_KEY[j]). The conversions seem to be entirely redundant.