Python – Copy object in Python

copyobjectpython

I'm new to Python, so I tried writing a class to emulate string functions in C:

class CString:

  def __init__(self,str):
        self.value=[]
        self.value.extend(list(str))

def strcpy(cstring1,cstring2):
      import copy
      cstring1=copy.copy(cstring2)

def puts(cstring1):
    print ''.join(cstring1.value)

But strcpy doesn't seem to be working:

>>obj1=CString("Hello World")
>>obj2=CString("Hai World!")
>>puts(obj1)
Hello World!
>>puts(obj2)
Hai World!
>>strcpy(obj1,obj2)
>>puts(obj1)
Hello World!

Am I assigning copy.copy(cstring2) wrongly?

Best Answer

The line

cstring1 = copy.copy(cstring2)

only changes the local variable called cstring1, it does not change the variable in the outer scope called obj1.

Have a look at this other stackoverflow question for more information.

Related Solutions

Javascript – Detecting an undefined object property

The usual way to check if the value of a property is the special value undefined, is:

if(o.myProperty === undefined) {
  alert("myProperty value is the special value `undefined`");
}

To check if an object does not actually have such a property, and will therefore return undefined by default when you try and access it:

if(!o.hasOwnProperty('myProperty')) {
  alert("myProperty does not exist");
}

To check if the value associated with an identifier is the special value undefined, or if that identifier has not been declared. Note: this method is the only way of referring to an undeclared (note: different from having a value of undefined) identifier without an early error:

if(typeof myVariable === 'undefined') {
  alert('myVariable is either the special value `undefined`, or it has not been declared');
}

In versions of JavaScript prior to ECMAScript 5, the property named "undefined" on the global object was writeable, and therefore a simple check foo === undefined might behave unexpectedly if it had accidentally been redefined. In modern JavaScript, the property is read-only.

However, in modern JavaScript, "undefined" is not a keyword, and so variables inside functions can be named "undefined" and shadow the global property.

If you are worried about this (unlikely) edge case, you can use the void operator to get at the special undefined value itself:

if(myVariable === void 0) {
  alert("myVariable is the special value `undefined`");
}

Python – How to execute a program or call a system command

Use the subprocess module in the standard library:

import subprocess
subprocess.run(["ls", "-l"])

The advantage of subprocess.run over os.system is that it is more flexible (you can get the stdout, stderr, the "real" status code, better error handling, etc...).

Even the documentation for os.system recommends using subprocess instead:

The subprocess module provides more powerful facilities for spawning new processes and retrieving their results; using that module is preferable to using this function. See the Replacing Older Functions with the subprocess Module section in the subprocess documentation for some helpful recipes.

On Python 3.4 and earlier, use subprocess.call instead of .run:

subprocess.call(["ls", "-l"])

Best Answer

Related Solutions

Javascript – Detecting an undefined object property

Python – How to execute a program or call a system command

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