Unlike in other languages like C#, there is no way to import an empty string from Python standard library. However, there is a built-in function str(), which defaults to an empty string when no argument was supplied.
What would be the pros and cons of using str()--compared to '' or empty_str = ''--to denote an empty string? I guess the overhead would be negligible, and it is as readable as '' or "". For one thing, it is visually clear that the variable is a str even in Notepad. One obvious disadvantage is that you have to type the keyboard a few more times than '' although not as many as empty_str. Can you think of any other merits or demerits?
It has exactly the same effect, and '' (or "") should be used (similar [] for list, and () for tuple, {} for dict).
And Python will indeed have an internal global cache for small constants (short strings, including the empty string, and small numbers). Check that by id('') or id(0), it will very likely always return the same. (This is implementation specific, but CPython will behave this way.)
Use s = ''.
First, Python itself is not the fastest language in universe, and Python developers are not supposed to make such kind of optimization, like creating global variables for empty constants or optimizing variable initialization with constants. Your code will be considered as not maintainable, because others would not understand your purpose.
Second, any static code analyzer will always understand the type of your variable from a constant. But when function is called, analyzer has to find the declaration and get the return value type. I dont speak about str() - as a builtin, it's return type is well known to analyzers. But in general approach is not good, because in Python programmers not always explicitly define the return value type of functions.
I was wondering if there is a proper Python convention to distinguish between functions that either alter their arguments in place or functions that leave their arguments in tact and return an altered copy. For example, consider two functions that apply some permutation. Function f takes a list as an argument and shuffles the elements around, while function g takes a list, copies it, applies f and then returns the altered copy.
For the above example, I figured that f could be called permute(x), while g could be permutation(x), using verbs and nouns to distinguish. This is not always really optimal, though, and in some cases it might lead to confusion as to whether an argument is going to get changed along the way - especially if f were to have some other value to return.
Is there a standard way to deal with this problem?
There is no handy naming convention, not written down in places like PEP-8 at any rate.
The Python standard library does use such a convention to a certain extend. Compare:
listobj.sort()
listobj.reverse()
with
sorted_listobj = sorted(listobj)
reversed_iterator = reversed(listobj)
Using a verb when acting on the object directly, an adjective when returning a new.
The convention isn't consistent. When enumerating an iterable, you use enumerate(), not enumerated(). filter() doesn't alter the input sequence in place. round() doesn't touch the input number object. compile() produces a new bytecode object. Etc. But none of those operations have in-place equivalents in the Python standard library anyway, and I am not aware of any use of adjectives where the input is altered in-place.
I am trying to learn Python and am a bit confused about a script I am playing with. I am using Python to launch scapy. There are some conditional statements that test for certain values. My confusion is centered around how the values are checked. I hope I am using the terms attributes and methods appropriately. I am still trying to figure out the builtin features vs. what is included with scapy. I've been using Powershell mainly for the last few years so its hard to switch gears :)
tcp_connect_scan_resp = sr1(IP(dst=dst_ip)/TCP(sport=src_port,dport=dst_port,flags="S"),timeout=10)
if(str(type(tcp_connect_scan_resp))=="<type 'NoneType'>"):
Print "Closed"
elif(tcp_connect_scan_resp.haslayer(TCP)):
if(tcp_connect_scan_resp.getlayer(TCP).flags == 0x12):
The first conditional statement appears to be check for the attribute 'type'. Why would they use the Python built-in str() and type() functions in this case? If I just use type() it pulls the same value.
For the second and third conditional statements appear to be using methods built into scapy. What is the logic for including the brackets () on the outside of the statements? Again if I run them manually, I get the proper value.
The second statement, the parantheses around the expression of an if statement, is simply unnecessary and bad style.
The first statement warrants a more detailed explanation:
if(str(type(tcp_connect_scan_resp))=="<type 'NoneType'>"):
This checks if the string representation of the type that tcp_connect_scan_resp is of is equal to "". This is a bad form of type checking, used in a bad way. There are situations where type checking may be necessary, but generally you should try to avoid it in Python (see duck typing). If you must, use isinstance().
In the case of the Python builtin type None, the idiomatic way is to just write
if foo is None
Now, the reason you got the "same result" by using type() yourself, is that if you enter someting in an interactive Python shell, the interpreter represents the value for you (by calling __repr__()). Except for basic types that have literal notations, like integers, strings, or sequences, the representation of an object isn't necessarlily the same as its value (or what you would type in to recreate that same object).
So, when you do
>>> foo = type(42)
>>> foo
<type 'int'>
the interpreter prints '<type 'int'>', but the result of the call is actualy int, the built-in type for integers:
>>> type(42) == int
True
>>> type(42) == "<type 'int'>"
False
Also, consider this:
Libraries or tools written to help with a specific field of expertise are often written by experts in those fields - not necessarily experts in Python. In my opinion, you often see this in scientific libraries (matplotlib and numpy for example). This doesn't mean they're bad libraries, but they often aren't a good inspiration for Pythonic style.
Never check a type by comparing str(type(obj)) == 'ClassName'.
You should use isinstance(obj, Class), or for None you just write if obj is None.
Why are there no ++ and -- operators in Python?
It's not because it doesn't make sense; it makes perfect sense to define "x++" as "x += 1, evaluating to the previous binding of x".
If you want to know the original reason, you'll have to either wade through old Python mailing lists or ask somebody who was there (eg. Guido), but it's easy enough to justify after the fact:
Simple increment and decrement aren't needed as much as in other languages. You don't write things like for(int i = 0; i < 10; ++i) in Python very often; instead you do things like for i in range(0, 10).
Since it's not needed nearly as often, there's much less reason to give it its own special syntax; when you do need to increment, += is usually just fine.
It's not a decision of whether it makes sense, or whether it can be done--it does, and it can. It's a question of whether the benefit is worth adding to the core syntax of the language. Remember, this is four operators--postinc, postdec, preinc, predec, and each of these would need to have its own class overloads; they all need to be specified, and tested; it would add opcodes to the language (implying a larger, and therefore slower, VM engine); every class that supports a logical increment would need to implement them (on top of += and -=).
This is all redundant with += and -=, so it would become a net loss.
This original answer I wrote is a myth from the folklore of computing: debunked by Dennis Ritchie as "historically impossible" as noted in the letters to the editors of Communications of the ACM July 2012 doi:10.1145/2209249.2209251
The C increment/decrement operators were invented at a time when the C compiler wasn't very smart and the authors wanted to be able to specify the direct intent that a machine language operator should be used which saved a handful of cycles for a compiler which might do a
load memory
load 1
add
store memory
instead of
inc memory
and the PDP-11 even supported "autoincrement" and "autoincrement deferred" instructions corresponding to *++p and *p++, respectively. See section 5.3 of the manual if horribly curious.
As compilers are smart enough to handle the high-level optimization tricks built into the syntax of C, they are just a syntactic convenience now.
Python doesn't have tricks to convey intentions to the assembler because it doesn't use one.
I always assumed it had to do with this line of the zen of python:
There should be one — and preferably only one — obvious way to do it.
x++ and x+=1 do the exact same thing, so there is no reason to have both.
Of course, we could say "Guido just decided that way", but I think the question is really about the reasons for that decision. I think there are several reasons:
It mixes together statements and expressions, which is not good practice. See http://norvig.com/python-iaq.html
It generally encourages people to write less readable code
Extra complexity in the language implementation, which is unnecessary in Python, as already mentioned
Because, in Python, integers are immutable (int's += actually returns a different object).
Also, with ++/-- you need to worry about pre- versus post- increment/decrement, and it takes only one more keystroke to write x+=1. In other words, it avoids potential confusion at the expense of very little gain.
Clarity!
Python is a lot about clarity and no programmer is likely to correctly guess the meaning of --a unless s/he's learned a language having that construct.
Python is also a lot about avoiding constructs that invite mistakes and the ++ operators are known to be rich sources of defects.
These two reasons are enough not to have those operators in Python.
The decision that Python uses indentation to mark blocks rather
than syntactical means such as some form of begin/end bracketing
or mandatory end marking is based largely on the same considerations.
For illustration, have a look at the discussion around introducing a conditional operator (in C: cond ? resultif : resultelse) into Python in 2005.
Read at least the first message and the decision message of that discussion (which had several precursors on the same topic previously).
Trivia:
The PEP frequently mentioned therein is the "Python Enhancement Proposal" PEP 308. LC means list comprehension, GE means generator expression (and don't worry if those confuse you, they are none of the few complicated spots of Python).
My understanding of why python does not have ++ operator is following: When you write this in python a=b=c=1 you will get three variables (labels) pointing at same object (which value is 1). You can verify this by using id function which will return an object memory address:
In [19]: id(a)
Out[19]: 34019256
In [20]: id(b)
Out[20]: 34019256
In [21]: id(c)
Out[21]: 34019256
All three variables (labels) point to the same object. Now increment one of variable and see how it affects memory addresses:
In [22] a = a + 1
In [23]: id(a)
Out[23]: 34019232
In [24]: id(b)
Out[24]: 34019256
In [25]: id(c)
Out[25]: 34019256
You can see that variable a now points to another object as variables b and c. Because you've used a = a + 1 it is explicitly clear. In other words you assign completely another object to label a. Imagine that you can write a++ it would suggest that you did not assign to variable a new object but ratter increment the old one. All this stuff is IMHO for minimization of confusion. For better understanding see how python variables works:
In Python, why can a function modify some arguments as perceived by the caller, but not others?
Is Python call-by-value or call-by-reference? Neither.
Does Python pass by value, or by reference?
Is Python pass-by-reference or pass-by-value?
Python: How do I pass a variable by reference?
Understanding Python variables and Memory Management
Emulating pass-by-value behaviour in python
Python functions call by reference
Code Like a Pythonista: Idiomatic Python
It was just designed that way. Increment and decrement operators are just shortcuts for x = x + 1. Python has typically adopted a design strategy which reduces the number of alternative means of performing an operation. Augmented assignment is the closest thing to increment/decrement operators in Python, and they weren't even added until Python 2.0.
I'm very new to python but I suspect the reason is because of the emphasis between mutable and immutable objects within the language. Now, I know that x++ can easily be interpreted as x = x + 1, but it LOOKS like you're incrementing in-place an object which could be immutable.
Just my guess/feeling/hunch.
To complete already good answers on that page:
Let's suppose we decide to do this, prefix (++i) that would break the unary + and - operators.
Today, prefixing by ++ or -- does nothing, because it enables unary plus operator twice (does nothing) or unary minus twice (twice: cancels itself)
>>> i=12
>>> ++i
12
>>> --i
12
So that would potentially break that logic.
now if one needs it for list comprehensions or lambdas, from python 3.8 it's possible with the new := assignment operator (PEP572)
pre-incrementing a and assign it to b:
>>> a = 1
>>> b = (a:=a+1)
>>> b
2
>>> a
2
post-incrementing just needs to make up the premature add by subtracting 1:
>>> a = 1
>>> b = (a:=a+1)-1
>>> b
1
>>> a
2
I believe it stems from the Python creed that "explicit is better than implicit".
First, Python is only indirectly influenced by C; it is heavily influenced by ABC, which apparently does not have these operators, so it should not be any great surprise not to find them in Python either.
Secondly, as others have said, increment and decrement are supported by += and -= already.
Third, full support for a ++ and -- operator set usually includes supporting both the prefix and postfix versions of them. In C and C++, this can lead to all kinds of "lovely" constructs that seem (to me) to be against the spirit of simplicity and straight-forwardness that Python embraces.
For example, while the C statement while(*t++ = *s++); may seem simple and elegant to an experienced programmer, to someone learning it, it is anything but simple. Throw in a mixture of prefix and postfix increments and decrements, and even many pros will have to stop and think a bit.
The ++ class of operators are expressions with side effects. This is something generally not found in Python.
For the same reason an assignment is not an expression in Python, thus preventing the common if (a = f(...)) { /* using a here */ } idiom.
Lastly I suspect that there operator are not very consistent with Pythons reference semantics. Remember, Python does not have variables (or pointers) with the semantics known from C/C++.
as i understood it so you won't think the value in memory is changed.
in c when you do x++ the value of x in memory changes.
but in python all numbers are immutable hence the address that x pointed as still has x not x+1. when you write x++ you would think that x change what really happens is that x refrence is changed to a location in memory where x+1 is stored or recreate this location if doe's not exists.
Other answers have described why it's not needed for iterators, but sometimes it is useful when assigning to increase a variable in-line, you can achieve the same effect using tuples and multiple assignment:
b = ++a becomes:
a,b = (a+1,)*2
and b = a++ becomes:
a,b = a+1, a
Python 3.8 introduces the assignment := operator, allowing us to achievefoo(++a) with
foo(a:=a+1)
foo(a++) is still elusive though.
Maybe a better question would be to ask why do these operators exist in C. K&R calls increment and decrement operators 'unusual' (Section 2.8page 46). The Introduction calls them 'more concise and often more efficient'. I suspect that the fact that these operations always come up in pointer manipulation also has played a part in their introduction.
In Python it has been probably decided that it made no sense to try to optimise increments (in fact I just did a test in C, and it seems that the gcc-generated assembly uses addl instead of incl in both cases) and there is no pointer arithmetic; so it would have been just One More Way to Do It and we know Python loathes that.
This may be because #GlennMaynard is looking at the matter as in comparison with other languages, but in Python, you do things the python way. It's not a 'why' question. It's there and you can do things to the same effect with x+=. In The Zen of Python, it is given: "there should only be one way to solve a problem." Multiple choices are great in art (freedom of expression) but lousy in engineering.
I think this relates to the concepts of mutability and immutability of objects. 2,3,4,5 are immutable in python. Refer to the image below. 2 has fixed id until this python process.
x++ would essentially mean an in-place increment like C. In C, x++ performs in-place increments. So, x=3, and x++ would increment 3 in the memory to 4, unlike python where 3 would still exist in memory.
Thus in python, you don't need to recreate a value in memory. This may lead to performance optimizations.
This is a hunch based answer.
I know this is an old thread, but the most common use case for ++i is not covered, that being manually indexing sets when there are no provided indices. This situation is why python provides enumerate()
Example : In any given language, when you use a construct like foreach to iterate over a set - for the sake of the example we'll even say it's an unordered set and you need a unique index for everything to tell them apart, say
i = 0
stuff = {'a': 'b', 'c': 'd', 'e': 'f'}
uniquestuff = {}
for key, val in stuff.items() :
uniquestuff[key] = '{0}{1}'.format(val, i)
i += 1
In cases like this, python provides an enumerate method, e.g.
for i, (key, val) in enumerate(stuff.items()) :
In addition to the other excellent answers here, ++ and -- are also notorious for undefined behavior. For example, what happens in this code?
foo[bar] = bar++;
It's so innocent-looking, but it's wrong C (and C++), because you don't know whether the first bar will have been incremented or not. One compiler might do it one way, another might do it another way, and a third might make demons fly out of your nose. All would be perfectly conformant with the C and C++ standards.
(EDIT: C++17 has changed the behavior of the given code so that it is defined; it will be equivalent to foo[bar+1] = bar; ++bar; — which nonetheless might not be what the programmer is expecting.)
Undefined behavior is seen as a necessary evil in C and C++, but in Python, it's just evil, and avoided as much as possible.