Valid memory address in Python

Question

I am looking for suggestions of improving this function to check whether a memory address is appropriate or not.

def isweird(addr, cpu_name):
 """Check if a memory address is weird
 Args:
 addr (str): a memory address
 cpu_name (str): cpu name
 Returns:
 bool: True if the address is weird
 """
 if not isinstance(addr, six.string_types):
 raise Exception('The memory address must be a string.')
 if addr == '0x0':
 return True
 addr = addr.lower()
 # Strip leading zeroes
 addr = addr[2:].lstrip('0')
 if utils.is64(cpu_name):
 if len(addr) <= 8:
 val = int(addr, 16)
 if val <= (1 << 16): # val <= 0xffff (ie: first 64k)
 return True
 elif addr.startswith('ffffffff'):
 addr = addr[8:] # 8 == len('ffffffff')
 val = int(addr, 16)
 if val >= ((1 << 32) - (1 << 16)): # val >= 0xffffffffffff0000 (ie: last 64k)
 return True
 else:
 val = int(addr, 16)
 if val <= 1 << 16: # val <= 0xffff (ie: first 64k)
 return True
 if val >= ((1 << 32) - (1 << 16)): # val >= 0xffff0000 (ie: last 64k)
 return True
 return False

Answer 1

In this code:

if len(addr) <= 8:
 val = int(addr, 16)
 if val <= (1 << 16): # val <= 0xffff (ie: first 64k)
 return True
elif addr.startswith('ffffffff'):
 addr = addr[8:] # 8 == len('ffffffff')
 val = int(addr, 16)
 if val >= ((1 << 32) - (1 << 16)): # val >= 0xffffffffffff0000 (ie: last 64k)
 return True

You can replace the inner if statements with return statements:

if len(addr) <= 8:
 val = int(addr, 16)
 return val <= (1 << 16) # val <= 0xffff (ie: first 64k)
elif addr.startswith('ffffffff'):
 addr = addr[8:] # 8 == len('ffffffff')
 val = int(addr, 16)
 return val >= ((1 << 32) - (1 << 16)) # val >= 0xffffffffffff0000 (ie: last 64k)

This is possible because if these conditions are false, the execution will reach the return False at the end of the function. The reason to rewrite this way is so that the reader doesn't have to follow the execution path until the end of the function, he can know on these lines that no further statements will be executed, the return value is already decided at these points.

---

Some of the comments are incorrect and misleading:

if val <= (1 << 16): # val <= 0xffff (ie: first 64k)
if val >= ((1 << 32) - (1 << 16)): # val >= 0xffffffffffff0000 (ie: last 64k)

1 << 16 is 0x10000, not 0xffff.

(1 << 32) - (1 << 16) is 0xffff0000, not 0xffffffffffff0000.

---

Following the naming conventions of Python, the function isweird should be named is_weird instead.

Answer 2

Don't use binary operators when you can use numbers. Take:

if val <= (1 << 16): # val <= 0xffff (ie: first 64k)

This can just be:

if val <= 0xffff:

This allows you to remove the comments, whilst keeping clarity for users to understand what numbers you are using.

• Caridorc: "boolean operators" I guess you meant "binary operators"
• user650881: Minor correction, 1<<16 == 0x10000