Design e Histórico FAQ

Sumário

Por que o Python usa indentação para agrupamento de declarações?

Guido van Rossum acredita que usar indentação para agrupamento é extremamente elegante e contribui muito para a clareza do programa Python. Muitas pessoas aprendem a amar esta ferramente depois de um tempo.

Uma vez que não há colchetes de início / fim, não pode haver um desacordo entre o agrupamento percebido pelo analisador e pelo leitor humano. Ocasionalmente, programadores C irão encontrar um fragmento de código como este:

if (x <= y)
        x++;
        y--;
z++;

Only the x++ statement is executed if the condition is true, but the indentation leads you to believe otherwise. Even experienced C programmers will sometimes stare at it a long time wondering why y is being decremented even for x > y.

Because there are no begin/end brackets, Python is much less prone to coding-style conflicts. In C there are many different ways to place the braces. If you’re used to reading and writing code that uses one style, you will feel at least slightly uneasy when reading (or being required to write) another style.

Many coding styles place begin/end brackets on a line by themselves. This makes programs considerably longer and wastes valuable screen space, making it harder to get a good overview of a program. Ideally, a function should fit on one screen (say, 20–30 lines). 20 lines of Python can do a lot more work than 20 lines of C. This is not solely due to the lack of begin/end brackets – the lack of declarations and the high-level data types are also responsible – but the indentation-based syntax certainly helps.

Por que eu estou recebendo resultados estranhos com simples operações aritméticas?

Veja a próxima questão.

Por que o calculo de pontos flutuantes são tão imprecisos?

Usuários frequentemente são surpresos por resultados como este:

>>> 1.2 - 1.0
0.19999999999999996

e pensam que isto é um bug do Python. Não é não. Isto tem pouco a ver com o Python, e muito mais a ver com como a estrutura da plataforma lida com números em ponto flutuante.

The float type in CPython uses a C double for storage. A float object’s value is stored in binary floating-point with a fixed precision (typically 53 bits) and Python uses C operations, which in turn rely on the hardware implementation in the processor, to perform floating-point operations. This means that as far as floating-point operations are concerned, Python behaves like many popular languages including C and Java.

Muitos números podem ser escritos facilmente em notação decimal, mas não podem ser expressados exatamente em ponto flutuante binário.

>>> x = 1.2

o valor armazenado para x é uma (ótima) aproximação para o valor decimal 1.2, mas não é exatamente igual. Em uma máquina típica, o valor real armazenado é:

1.0011001100110011001100110011001100110011001100110011 (binary)

que é exatamente:

1.1999999999999999555910790149937383830547332763671875 (decimal)

The typical precision of 53 bits provides Python floats with 15–16 decimal digits of accuracy.

For a fuller explanation, please see the floating point arithmetic chapter in the Python tutorial.

Por que strings do Python são imutáveis?

Existem várias vantagens.

One is performance: knowing that a string is immutable means we can allocate space for it at creation time, and the storage requirements are fixed and unchanging. This is also one of the reasons for the distinction between tuples and lists.

Another advantage is that strings in Python are considered as “elemental” as numbers. No amount of activity will change the value 8 to anything else, and in Python, no amount of activity will change the string “eight” to anything else.

Por que o ‘self’ deve ser usado explicitamente em definições de método e chamadas?

A ideia foi emprestada do Modula-2. Acontece dela ser muito útil, por vários motivos.

First, it’s more obvious that you are using a method or instance attribute instead of a local variable. Reading self.x or self.meth() makes it absolutely clear that an instance variable or method is used even if you don’t know the class definition by heart. In C++, you can sort of tell by the lack of a local variable declaration (assuming globals are rare or easily recognizable) – but in Python, there are no local variable declarations, so you’d have to look up the class definition to be sure. Some C++ and Java coding standards call for instance attributes to have an m_ prefix, so this explicitness is still useful in those languages, too.

Second, it means that no special syntax is necessary if you want to explicitly reference or call the method from a particular class. In C++, if you want to use a method from a base class which is overridden in a derived class, you have to use the :: operator – in Python you can write baseclass.methodname(self, <argument list>). This is particularly useful for __init__() methods, and in general in cases where a derived class method wants to extend the base class method of the same name and thus has to call the base class method somehow.

Finally, for instance variables it solves a syntactic problem with assignment: since local variables in Python are (by definition!) those variables to which a value is assigned in a function body (and that aren’t explicitly declared global), there has to be some way to tell the interpreter that an assignment was meant to assign to an instance variable instead of to a local variable, and it should preferably be syntactic (for efficiency reasons). C++ does this through declarations, but Python doesn’t have declarations and it would be a pity having to introduce them just for this purpose. Using the explicit self.var solves this nicely. Similarly, for using instance variables, having to write self.var means that references to unqualified names inside a method don’t have to search the instance’s directories. To put it another way, local variables and instance variables live in two different namespaces, and you need to tell Python which namespace to use.

Por que não posso usar uma atribuição em uma expressão?

Many people used to C or Perl complain that they want to use this C idiom:

while (line = readline(f)) {
    // do something with line
}

where in Python you’re forced to write this:

while True:
    line = f.readline()
    if not line:
        break
    ...  # do something with line

The reason for not allowing assignment in Python expressions is a common, hard-to-find bug in those other languages, caused by this construct:

if (x = 0) {
    // error handling
}
else {
    // code that only works for nonzero x
}

The error is a simple typo: x = 0, which assigns 0 to the variable x, was written while the comparison x == 0 is certainly what was intended.

Many alternatives have been proposed. Most are hacks that save some typing but use arbitrary or cryptic syntax or keywords, and fail the simple criterion for language change proposals: it should intuitively suggest the proper meaning to a human reader who has not yet been introduced to the construct.

An interesting phenomenon is that most experienced Python programmers recognize the while True idiom and don’t seem to be missing the assignment in expression construct much; it’s only newcomers who express a strong desire to add this to the language.

There’s an alternative way of spelling this that seems attractive but is generally less robust than the “while True” solution:

line = f.readline()
while line:
    ...  # do something with line...
    line = f.readline()

The problem with this is that if you change your mind about exactly how you get the next line (e.g. you want to change it into sys.stdin.readline()) you have to remember to change two places in your program – the second occurrence is hidden at the bottom of the loop.

The best approach is to use iterators, making it possible to loop through objects using the for statement. For example, file objects support the iterator protocol, so you can write simply:

for line in f:
    ...  # do something with line...

Por que o Python usa métodos para algumas funcionalidades (ex: list.index()) mas funções para outras (ex: len(list))?

Como Guido disse:

(a) For some operations, prefix notation just reads better than postfix – prefix (and infix!) operations have a long tradition in mathematics which likes notations where the visuals help the mathematician thinking about a problem. Compare the easy with which we rewrite a formula like x*(a+b) into x*a + x*b to the clumsiness of doing the same thing using a raw OO notation.

(b) When I read code that says len(x) I know that it is asking for the length of something. This tells me two things: the result is an integer, and the argument is some kind of container. To the contrary, when I read x.len(), I have to already know that x is some kind of container implementing an interface or inheriting from a class that has a standard len(). Witness the confusion we occasionally have when a class that is not implementing a mapping has a get() or keys() method, or something that isn’t a file has a write() method.

—https://mail.python.org/pipermail/python-3000/2006-November/004643.html

Por que o join() é um método de string em vez de ser um método de lista ou tupla?

Strings se tornaram muito parecidas com outros tipos padrão a partir do Python 1.6, quando métodos que dão a mesma funcionalidade que sempre esteve disponível utilizando as funções do módulo de string foram adicionados. A maior parte desses novos métodos foram amplamente aceitos, mas o que parece deixar alguns programadores desconfortáveis é:

", ".join(['1', '2', '4', '8', '16'])

que dá o resultado:

"1, 2, 4, 8, 16"

Existem dois argumentos comuns contra esse uso.

The first runs along the lines of: “It looks really ugly using a method of a string literal (string constant)”, to which the answer is that it might, but a string literal is just a fixed value. If the methods are to be allowed on names bound to strings there is no logical reason to make them unavailable on literals.

The second objection is typically cast as: “I am really telling a sequence to join its members together with a string constant”. Sadly, you aren’t. For some reason there seems to be much less difficulty with having split() as a string method, since in that case it is easy to see that

"1, 2, 4, 8, 16".split(", ")

is an instruction to a string literal to return the substrings delimited by the given separator (or, by default, arbitrary runs of white space).

join() is a string method because in using it you are telling the separator string to iterate over a sequence of strings and insert itself between adjacent elements. This method can be used with any argument which obeys the rules for sequence objects, including any new classes you might define yourself. Similar methods exist for bytes and bytearray objects.

O quão rápidas são as exceções?

Um bloco de try/except é extremamente eficiente se nenhuma exceção for levantada. Na verdade, capturar uma exceção custa caro. Em versões do Python anteriores a 2.0 era como utilizar esse idioma:

try:
    value = mydict[key]
except KeyError:
    mydict[key] = getvalue(key)
    value = mydict[key]

Isso somente fazia sentido quando você esperava que o dicionário tivesse uma chave quase que toda vez. Se esse não fosse o caso, você escrevia desta maneira:

if key in mydict:
    value = mydict[key]
else:
    value = mydict[key] = getvalue(key)

For this specific case, you could also use value = dict.setdefault(key, getvalue(key)), but only if the getvalue() call is cheap enough because it is evaluated in all cases.

Por que não existe uma instrução de switch ou case no Python?

You can do this easily enough with a sequence of if... elif... elif... else. There have been some proposals for switch statement syntax, but there is no consensus (yet) on whether and how to do range tests. See PEP 275 for complete details and the current status.

For cases where you need to choose from a very large number of possibilities, you can create a dictionary mapping case values to functions to call. For example:

def function_1(...):
    ...

functions = {'a': function_1,
             'b': function_2,
             'c': self.method_1, ...}

func = functions[value]
func()

For calling methods on objects, you can simplify yet further by using the getattr() built-in to retrieve methods with a particular name:

def visit_a(self, ...):
    ...
...

def dispatch(self, value):
    method_name = 'visit_' + str(value)
    method = getattr(self, method_name)
    method()

It’s suggested that you use a prefix for the method names, such as visit_ in this example. Without such a prefix, if values are coming from an untrusted source, an attacker would be able to call any method on your object.

Can’t you emulate threads in the interpreter instead of relying on an OS-specific thread implementation?

Answer 1: Unfortunately, the interpreter pushes at least one C stack frame for each Python stack frame. Also, extensions can call back into Python at almost random moments. Therefore, a complete threads implementation requires thread support for C.

Answer 2: Fortunately, there is Stackless Python, which has a completely redesigned interpreter loop that avoids the C stack.

Por que expressões lambda não podem conter instruções?

Expressões lambda no Python não podem conter instruções porque o framework sintático do Python não consegue lidar com instruções aninhadas dentro de expressões. No entanto, no Python, isso não é um problema sério. Diferentemente das formas de lambda em outras linguagens, onde elas adicionam funcionalidade, lambdas de Python são apenas notações simplificadas se você tiver muita preguiça de definir uma função.

Funções já são objetos de primeira classe em Python, e podem ser declaradas em um escopo local. Portanto a única vantagem de usar um lambda em vez de uma função definida localmente é que você não precisa inventar um nome para a função – mas esta só é uma variável local para a qual o objeto da função (que é exatamente do mesmo tipo de um objeto que uma expressão lambda carrega) é atribuído.

O Python pode ser compilado para linguagem de máquina, C ou alguma outra linguagem?

Cython compiles a modified version of Python with optional annotations into C extensions. Nuitka is an up-and-coming compiler of Python into C++ code, aiming to support the full Python language. For compiling to Java you can consider VOC.

Como o Python gerencia memória?

The details of Python memory management depend on the implementation. The standard implementation of Python, CPython, uses reference counting to detect inaccessible objects, and another mechanism to collect reference cycles, periodically executing a cycle detection algorithm which looks for inaccessible cycles and deletes the objects involved. The gc module provides functions to perform a garbage collection, obtain debugging statistics, and tune the collector’s parameters.

Other implementations (such as Jython or PyPy), however, can rely on a different mechanism such as a full-blown garbage collector. This difference can cause some subtle porting problems if your Python code depends on the behavior of the reference counting implementation.

In some Python implementations, the following code (which is fine in CPython) will probably run out of file descriptors:

for file in very_long_list_of_files:
    f = open(file)
    c = f.read(1)

Indeed, using CPython’s reference counting and destructor scheme, each new assignment to f closes the previous file. With a traditional GC, however, those file objects will only get collected (and closed) at varying and possibly long intervals.

If you want to write code that will work with any Python implementation, you should explicitly close the file or use the with statement; this will work regardless of memory management scheme:

for file in very_long_list_of_files:
    with open(file) as f:
        c = f.read(1)

Por que o CPython não usa uma forma mais tradicional de esquema de coleta de lixo?

For one thing, this is not a C standard feature and hence it’s not portable. (Yes, we know about the Boehm GC library. It has bits of assembler code for most common platforms, not for all of them, and although it is mostly transparent, it isn’t completely transparent; patches are required to get Python to work with it.)

Traditional GC also becomes a problem when Python is embedded into other applications. While in a standalone Python it’s fine to replace the standard malloc() and free() with versions provided by the GC library, an application embedding Python may want to have its own substitute for malloc() and free(), and may not want Python’s. Right now, CPython works with anything that implements malloc() and free() properly.

Por que toda memória não é liberada quando o CPython fecha?

Objects referenced from the global namespaces of Python modules are not always deallocated when Python exits. This may happen if there are circular references. There are also certain bits of memory that are allocated by the C library that are impossible to free (e.g. a tool like Purify will complain about these). Python is, however, aggressive about cleaning up memory on exit and does try to destroy every single object.

If you want to force Python to delete certain things on deallocation use the atexit module to run a function that will force those deletions.

Por que existem tipos de dados separados para tuplas e listas?

Lists and tuples, while similar in many respects, are generally used in fundamentally different ways. Tuples can be thought of as being similar to Pascal records or C structs; they’re small collections of related data which may be of different types which are operated on as a group. For example, a Cartesian coordinate is appropriately represented as a tuple of two or three numbers.

Lists, on the other hand, are more like arrays in other languages. They tend to hold a varying number of objects all of which have the same type and which are operated on one-by-one. For example, os.listdir('.') returns a list of strings representing the files in the current directory. Functions which operate on this output would generally not break if you added another file or two to the directory.

Tuples are immutable, meaning that once a tuple has been created, you can’t replace any of its elements with a new value. Lists are mutable, meaning that you can always change a list’s elements. Only immutable elements can be used as dictionary keys, and hence only tuples and not lists can be used as keys.

Como as listas são implementadas no CPython?

CPython’s lists are really variable-length arrays, not Lisp-style linked lists. The implementation uses a contiguous array of references to other objects, and keeps a pointer to this array and the array’s length in a list head structure.

This makes indexing a list a[i] an operation whose cost is independent of the size of the list or the value of the index.

When items are appended or inserted, the array of references is resized. Some cleverness is applied to improve the performance of appending items repeatedly; when the array must be grown, some extra space is allocated so the next few times don’t require an actual resize.

Como são os dicionários implementados no CPython?

CPython’s dictionaries are implemented as resizable hash tables. Compared to B-trees, this gives better performance for lookup (the most common operation by far) under most circumstances, and the implementation is simpler.

Dictionaries work by computing a hash code for each key stored in the dictionary using the hash() built-in function. The hash code varies widely depending on the key and a per-process seed; for example, “Python” could hash to -539294296 while “python”, a string that differs by a single bit, could hash to 1142331976. The hash code is then used to calculate a location in an internal array where the value will be stored. Assuming that you’re storing keys that all have different hash values, this means that dictionaries take constant time – O(1), in Big-O notation – to retrieve a key.

Por que chaves de dicionário devem ser imutáveis?

The hash table implementation of dictionaries uses a hash value calculated from the key value to find the key. If the key were a mutable object, its value could change, and thus its hash could also change. But since whoever changes the key object can’t tell that it was being used as a dictionary key, it can’t move the entry around in the dictionary. Then, when you try to look up the same object in the dictionary it won’t be found because its hash value is different. If you tried to look up the old value it wouldn’t be found either, because the value of the object found in that hash bin would be different.

If you want a dictionary indexed with a list, simply convert the list to a tuple first; the function tuple(L) creates a tuple with the same entries as the list L. Tuples are immutable and can therefore be used as dictionary keys.

Algumas soluções inaceitáveis que foram propostas:

  • Hash lists by their address (object ID). This doesn’t work because if you construct a new list with the same value it won’t be found; e.g.:

    mydict = {[1, 2]: '12'}
print(mydict[[1, 2]])

    would raise a KeyError exception because the id of the [1, 2] used in the second line differs from that in the first line. In other words, dictionary keys should be compared using ==, not using is.

  • Make a copy when using a list as a key. This doesn’t work because the list, being a mutable object, could contain a reference to itself, and then the copying code would run into an infinite loop.

  • Allow lists as keys but tell the user not to modify them. This would allow a class of hard-to-track bugs in programs when you forgot or modified a list by accident. It also invalidates an important invariant of dictionaries: every value in d.keys() is usable as a key of the dictionary.

  • Mark lists as read-only once they are used as a dictionary key. The problem is that it’s not just the top-level object that could change its value; you could use a tuple containing a list as a key. Entering anything as a key into a dictionary would require marking all objects reachable from there as read-only – and again, self-referential objects could cause an infinite loop.

There is a trick to get around this if you need to, but use it at your own risk: You can wrap a mutable structure inside a class instance which has both a __eq__() and a __hash__() method. You must then make sure that the hash value for all such wrapper objects that reside in a dictionary (or other hash based structure), remain fixed while the object is in the dictionary (or other structure).

class ListWrapper:
    def __init__(self, the_list):
        self.the_list = the_list

    def __eq__(self, other):
        return self.the_list == other.the_list

    def __hash__(self):
        l = self.the_list
        result = 98767 - len(l)*555
        for i, el in enumerate(l):
            try:
                result = result + (hash(el) % 9999999) * 1001 + i
            except Exception:
                result = (result % 7777777) + i * 333
        return result

Note that the hash computation is complicated by the possibility that some members of the list may be unhashable and also by the possibility of arithmetic overflow.

Furthermore it must always be the case that if o1 == o2 (ie o1.__eq__(o2) is True) then hash(o1) == hash(o2) (ie, o1.__hash__() == o2.__hash__()), regardless of whether the object is in a dictionary or not. If you fail to meet these restrictions dictionaries and other hash based structures will misbehave.

In the case of ListWrapper, whenever the wrapper object is in a dictionary the wrapped list must not change to avoid anomalies. Don’t do this unless you are prepared to think hard about the requirements and the consequences of not meeting them correctly. Consider yourself warned.

Por que lista.sort() não retorna a lista ordenada?

Em situações nas quais performance importa, fazer uma cópia da lista só para ordenar seria desperdício. Portanto, list.sort() ordena a lista. De forma a lembrá-lo desse fato, isso não retorna a lista ordenada. Desta forma, você não vai ser confundido a acidentalmente sobrescrever uma lista quando você precisar de uma cópia ordenada mas também precisar manter a versão não ordenada.

Se você quiser retornar uma nova lista, use a função embutida sorted() ao invés. Essa função cria uma nova lista a partir de um iterável provido, o ordena e retorna. Por exemplo, aqui é como se itera em cima das chaves de um dicionário de maneira ordenada:

for key in sorted(mydict):
    ...  # do whatever with mydict[key]...

How do you specify and enforce an interface spec in Python?

An interface specification for a module as provided by languages such as C++ and Java describes the prototypes for the methods and functions of the module. Many feel that compile-time enforcement of interface specifications helps in the construction of large programs.

Python 2.6 adds an abc module that lets you define Abstract Base Classes (ABCs). You can then use isinstance() and issubclass() to check whether an instance or a class implements a particular ABC. The collections.abc module defines a set of useful ABCs such as Iterable, Container, and MutableMapping.

For Python, many of the advantages of interface specifications can be obtained by an appropriate test discipline for components. There is also a tool, PyChecker, which can be used to find problems due to subclassing.

A good test suite for a module can both provide a regression test and serve as a module interface specification and a set of examples. Many Python modules can be run as a script to provide a simple “self test.” Even modules which use complex external interfaces can often be tested in isolation using trivial “stub” emulations of the external interface. The doctest and unittest modules or third-party test frameworks can be used to construct exhaustive test suites that exercise every line of code in a module.

An appropriate testing discipline can help build large complex applications in Python as well as having interface specifications would. In fact, it can be better because an interface specification cannot test certain properties of a program. For example, the append() method is expected to add new elements to the end of some internal list; an interface specification cannot test that your append() implementation will actually do this correctly, but it’s trivial to check this property in a test suite.

Writing test suites is very helpful, and you might want to design your code with an eye to making it easily tested. One increasingly popular technique, test-directed development, calls for writing parts of the test suite first, before you write any of the actual code. Of course Python allows you to be sloppy and not write test cases at all.

Why is there no goto?

You can use exceptions to provide a “structured goto” that even works across function calls. Many feel that exceptions can conveniently emulate all reasonable uses of the “go” or “goto” constructs of C, Fortran, and other languages. For example:

class label(Exception): pass  # declare a label

try:
    ...
    if condition: raise label()  # goto label
    ...
except label:  # where to goto
    pass
...

This doesn’t allow you to jump into the middle of a loop, but that’s usually considered an abuse of goto anyway. Use sparingly.

Por que strings brutas (r-strings) não podem terminar com uma barra invertida?

More precisely, they can’t end with an odd number of backslashes: the unpaired backslash at the end escapes the closing quote character, leaving an unterminated string.

Raw strings were designed to ease creating input for processors (chiefly regular expression engines) that want to do their own backslash escape processing. Such processors consider an unmatched trailing backslash to be an error anyway, so raw strings disallow that. In return, they allow you to pass on the string quote character by escaping it with a backslash. These rules work well when r-strings are used for their intended purpose.

If you’re trying to build Windows pathnames, note that all Windows system calls accept forward slashes too:

f = open("/mydir/file.txt")  # works fine!

If you’re trying to build a pathname for a DOS command, try e.g. one of

dir = r"\this\is\my\dos\dir" "\\"
dir = r"\this\is\my\dos\dir\ "[:-1]
dir = "\\this\\is\\my\\dos\\dir\\"

Por que o Python não tem uma instrução “with” para atribuição de atributos?

Python has a ‘with’ statement that wraps the execution of a block, calling code on the entrance and exit from the block. Some language have a construct that looks like this:

with obj:
    a = 1               # equivalent to obj.a = 1
    total = total + 1   # obj.total = obj.total + 1

In Python, such a construct would be ambiguous.

Outras linguagens, como Object Pascal, Delphi, e C++, usam tipos estáticos, então é possível saber, de maneira não ambígua, que membro está sendo atribuído. Esse é o principal ponto da tipagem estática – o compilador sempre sabe o escopo de toda variável em tempo de compilação.

O Python usa tipos dinâmicos. É impossível saber com antecedência que atributo vai ser referenciado em tempo de execução. Atributos membro podem ser adicionados ou removidos de objetos dinamicamente. Isso torna impossível saber, de uma leitura simples, que atributo está sendo referenciado: um atributo local, um atributo global ou um atributo membro?

For instance, take the following incomplete snippet:

def foo(a):
    with a:
        print(x)

The snippet assumes that “a” must have a member attribute called “x”. However, there is nothing in Python that tells the interpreter this. What should happen if “a” is, let us say, an integer? If there is a global variable named “x”, will it be used inside the with block? As you see, the dynamic nature of Python makes such choices much harder.

O benefício primário do “with” e funcionalidades similares da linguagem (redução de volume de código) pode, entretanto, ser facilmente alcançado no Python por atribuição. Em vez de:

function(args).mydict[index][index].a = 21
function(args).mydict[index][index].b = 42
function(args).mydict[index][index].c = 63

escreva isso:

ref = function(args).mydict[index][index]
ref.a = 21
ref.b = 42
ref.c = 63

Isso também tem o efeito colateral de aumentar a velocidade de execução por que ligações de nome são resolvidas a tempo de execução em Python, e a segunda versão só precisa performar a resolução uma vez.

Por que dois pontos são necessários para as instruções de if/while/def/class?

Os dois pontos são obrigatórios primeiramente para melhorar a leitura (um dos resultados da linguagem experimental ABC). Considere isso:

if a == b
    print(a)

versus

if a == b:
    print(a)

Note como a segunda é ligeiramente mais fácil de ler. Note com mais atenção como os dois pontos iniciam o exemplo nessa resposta de perguntas frequentes; é um uso padrão em Português.

Outro motivo menor é que os dois pontos deixam mais fácil para os editores com realce de sintaxe; eles podem procurar por dois pontos para decidir quando indentação precisa ser aumentada em vez de precisarem fazer uma análise mais elaborada do texto do programa.

Por que o Python permite vírgulas ao final de listas e tuplas?

O Python deixa você adicionar uma vírgula ao final de listas, tuplas e dicionários:

[1, 2, 3,]
('a', 'b', 'c',)
d = {
    "A": [1, 5],
    "B": [6, 7],  # last trailing comma is optional but good style
}

Existem várias razões para permitir isso.

Quando você possui um valor literal para uma lista, tupla, ou dicionário disposta através de múltiplas linhas, é mais fácil adicionar mais elementos porque você não precisa lembrar de adicionar uma vírgula na linha anterior. As linhas também podem ser reordenadas sem criar um erro de sintaxe.

Acidentalmente omitir a vírgula pode levar a erros que são difíceis de diagnosticar. Por exemplo:

x = [
  "fee",
  "fie"
  "foo",
  "fum"
]

Essa lista parece ter quatro elementos, mas na verdade contém três: “fee”, “fiefoo” e “fum”. Sempre adicionar a vírgula evita essa fonte de erro.

Permitir a vírgula no final também pode deixar a geração de código programático mais fácil.