Why does Python string fail to compare correctly?

Question

I am comparing two strings in Python. But the comparison fails to see that the values of variables firmware and hardware are the same as the strings "firmware" and "hardware".

gfirmware = create_string_buffer(str.encode("firmware"), 100)
ghardware = create_string_buffer(str.encode("hardware"), 100)
firmware = str(gfirmware,'utf-8')
hardware = str(ghardware,'utf-8')

print('firmware var = ' + firmware)
print('hardware var = ' + hardware)
print("\n")
print('firmware type = ' + str(type(firmware)))
print('hardware type = ' + str(type(hardware)))
print('"firmware" type = ' + str(type("firmware")))
print('"hardware" type = ' + str(type("hardware")))

print("Is it true? " + str(firmware != "firmware" and hardware != "hardware"))

Output:

firmware var = firmware
hardware var = hardware

firmware type = <class 'str'>
hardware type = <class 'str'>
"firmware" type = <class 'str'>
"hardware" type = <class 'str'>
Is it true? True

The values and the types of the variables and the strings are the same, as can be seen in the output.

So why does the comparison firmware != "firmware" and hardware != "hardware" return True, it should be returning False?

Note: I am intentionally using create_string_buffer() because I am passing gfirmware and ghardware into a C function. But this issue occurs even though I am not passing the variables into a C function.

I have looked at the following and other posts, but their issues were that the programmer was using the keyword is when they should have been using ==.

Why does comparing strings using either '==' or 'is' sometimes produce a different result?

python fails to compare strings

Strange behavior when comparing unicode objects with string objects

Mark · Accepted Answer · 2019-07-31 00:49:07Z

4

Your gfirmware and ghardware objects are large character buffers. When you convert them to strings with str(gfirmware,'utf-8') you get large strings:

>>> len(str(gfirmware, 'utf-8'))
100

because you still have all the padding.

You can use the value property on the buffers before converting to a string:

>> firmware = str(gfirmware.value,'utf-8')
>> hardware = str(ghardware.value,'utf-8')
>> firmware != "firmware", hardware != "hardware"
(False, False)

answered Jul 31, 2019 at 0:49

Mark

92.7k8 gold badges116 silver badges156 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

Fred Over a year ago

It didn't occur to me that it would be comparing padding as well. The C programmer in me thinks 0 as string termination character.

Vitaliy Terziev Over a year ago

ah.. and I thought that this fn was just some user defined code w/o any connection to the problem altough it crossed my mind for a second... now I see this is python lib/ foreign func

Mark Over a year ago

Yeah @Fred — it doesn't help that print(str(gfirmware,'utf-8')) shows you what looks like a nice normal string. print(repr(str(gfirmware,'utf-8'))) will show you the repr versions in all its padded glory.

Karl Knechtel Over a year ago

This is why it's important to debug properly before asking.

Collectives™ on Stack Overflow

Why does Python string fail to compare correctly?

1 Answer 1

4 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

4 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related