Использование флага (переменная булевого типа) для начинающих
Например есть такая задача перебрать данные и определить есть ли там нужные нам для примера возьмем ситуацию из жизни: переберем мешок с картошкой, нужно узнать есть ли там гнилая мешок представим списком [] из 1 и 2, где 1 — означает гнилую картофелину.
Как бы мы это делали? сначала мы бы взяли одну картофелину и посмотрели на нее, и так делали бы до конца или пока не встретили гнилую.
А в конце основываясь на наблюдениях делали бы выводы.
На словах все понятно, но когда смотришь код тех кто только еще познает питона, то в коде они так и норовят делать вывод о присутствии(или о том что нету гнилых) сразу на первой же картошке.
Пример ошибочного поведения в коде:
Вот тут в блоке else и кроется ошибка, т. е. мы еще не закончили перебирать мешок, а выводы уже делаем.
Попробуем решить с флагом:
А как же можно научиться пользоваться флагом?
Мне например помог счетчик…
Пример: посчитаем гнилую картошку:
Следующий шаг на пути счетчик-флаг понимание того что нам не нужно считать, а достаточно изменить счетчик с 0 на любое число, например 1
Но наш счетчик уже не счетчик, у него 2 возможных значения (0, 1)
А это прям подходит для булевых переменных, которые и являются классическим флагом со значениями (True, False)
Вот мы сменили значения 0 на False, 1 на True
Теперь если сменить имя счетчика count на rot_flag, то получится первый вариант решения, т. е. с полноценным флагом.
Это не идеальная задача.
На этой задаче я попытался показать его работу и как его понять можно с помощью счетчика, считать перебор многим дается легче на восприятии чем понятие «индикатор» или флаг.
Flag python что это
Flag variable is used as a signal in programming to let the program know that a certain condition has met. It usually acts as a boolean variable indicating a condition to be either true or false.
Example 1: Check if an array has any even number.
Input : arr[] = <1, 3, 7, 5>
Output : No All numbers are odd.
Input : arr[] = <1, 2, 7, 5>
Output : Yes There is one even number in the array.
We initialize a flag variable as false, then traverse the array. As soon as we find an even element, we set flag as true and break the loop. Finally we return flag.
Enum HOWTO¶
An Enum is a set of symbolic names bound to unique values. They are similar to global variables, but they offer a more useful repr() , grouping, type-safety, and a few other features.
They are most useful when you have a variable that can take one of a limited selection of values. For example, the days of the week:
Or perhaps the RGB primary colors:
As you can see, creating an Enum is as simple as writing a class that inherits from Enum itself.
Case of Enum Members
Because Enums are used to represent constants we recommend using UPPER_CASE names for members, and will be using that style in our examples.
Depending on the nature of the enum a member’s value may or may not be important, but either way that value can be used to get the corresponding member:
As you can see, the repr() of a member shows the enum name, the member name, and the value. The str() of a member shows only the enum name and member name:
The type of an enumeration member is the enum it belongs to:
Enum members have an attribute that contains just their name :
Likewise, they have an attribute for their value :
Unlike many languages that treat enumerations solely as name/value pairs, Python Enums can have behavior added. For example, datetime.date has two methods for returning the weekday: weekday() and isoweekday() . The difference is that one of them counts from 0-6 and the other from 1-7. Rather than keep track of that ourselves we can add a method to the Weekday enum to extract the day from the date instance and return the matching enum member:
The complete Weekday enum now looks like this:
Now we can find out what today is! Observe:
Of course, if you’re reading this on some other day, you’ll see that day instead.
This Weekday enum is great if our variable only needs one day, but what if we need several? Maybe we’re writing a function to plot chores during a week, and don’t want to use a list – we could use a different type of Enum :
We’ve changed two things: we’re inherited from Flag , and the values are all powers of 2.
Just like the original Weekday enum above, we can have a single selection:
But Flag also allows us to combine several members into a single variable:
You can even iterate over a Flag variable:
Okay, let’s get some chores set up:
And a function to display the chores for a given day:
In cases where the actual values of the members do not matter, you can save yourself some work and use auto() for the values:
Programmatic access to enumeration members and their attributes¶
Sometimes it’s useful to access members in enumerations programmatically (i.e. situations where Color.RED won’t do because the exact color is not known at program-writing time). Enum allows such access:
If you want to access enum members by name, use item access:
If you have an enum member and need its name or value :
Duplicating enum members and values¶
Having two enum members with the same name is invalid:
However, an enum member can have other names associated with it. Given two entries A and B with the same value (and A defined first), B is an alias for the member A . By-value lookup of the value of A will return the member A . By-name lookup of A will return the member A . By-name lookup of B will also return the member A :
Attempting to create a member with the same name as an already defined attribute (another member, a method, etc.) or attempting to create an attribute with the same name as a member is not allowed.
Ensuring unique enumeration values¶
By default, enumerations allow multiple names as aliases for the same value. When this behavior isn’t desired, you can use the unique() decorator:
Using automatic values¶
If the exact value is unimportant you can use auto :
The values are chosen by _generate_next_value_() , which can be overridden:
The _generate_next_value_() method must be defined before any members.
Iteration¶
Iterating over the members of an enum does not provide the aliases:
Note that the aliases Shape.ALIAS_FOR_SQUARE and Weekday.WEEKEND aren’t shown.
The special attribute __members__ is a read-only ordered mapping of names to members. It includes all names defined in the enumeration, including the aliases:
The __members__ attribute can be used for detailed programmatic access to the enumeration members. For example, finding all the aliases:
Aliases for flags include values with multiple flags set, such as 3 , and no flags set, i.e. 0 .
Comparisons¶
Enumeration members are compared by identity:
Ordered comparisons between enumeration values are not supported. Enum members are not integers (but see IntEnum below):
Equality comparisons are defined though:
Comparisons against non-enumeration values will always compare not equal (again, IntEnum was explicitly designed to behave differently, see below):
Allowed members and attributes of enumerations¶
Most of the examples above use integers for enumeration values. Using integers is short and handy (and provided by default by the Functional API), but not strictly enforced. In the vast majority of use-cases, one doesn’t care what the actual value of an enumeration is. But if the value is important, enumerations can have arbitrary values.
Enumerations are Python classes, and can have methods and special methods as usual. If we have this enumeration:
The rules for what is allowed are as follows: names that start and end with a single underscore are reserved by enum and cannot be used; all other attributes defined within an enumeration will become members of this enumeration, with the exception of special methods ( __str__() , __add__() , etc.), descriptors (methods are also descriptors), and variable names listed in _ignore_ .
Note: if your enumeration defines __new__() and/or __init__() then any value(s) given to the enum member will be passed into those methods. See Planet for an example.
Restricted Enum subclassing¶
A new Enum class must have one base enum class, up to one concrete data type, and as many object -based mixin classes as needed. The order of these base classes is:
Also, subclassing an enumeration is allowed only if the enumeration does not define any members. So this is forbidden:
But this is allowed:
Allowing subclassing of enums that define members would lead to a violation of some important invariants of types and instances. On the other hand, it makes sense to allow sharing some common behavior between a group of enumerations. (See OrderedEnum for an example.)
Pickling¶
Enumerations can be pickled and unpickled:
The usual restrictions for pickling apply: picklable enums must be defined in the top level of a module, since unpickling requires them to be importable from that module.
With pickle protocol version 4 it is possible to easily pickle enums nested in other classes.
It is possible to modify how enum members are pickled/unpickled by defining __reduce_ex__() in the enumeration class.
Functional API¶
The Enum class is callable, providing the following functional API:
The semantics of this API resemble namedtuple . The first argument of the call to Enum is the name of the enumeration.
The second argument is the source of enumeration member names. It can be a whitespace-separated string of names, a sequence of names, a sequence of 2-tuples with key/value pairs, or a mapping (e.g. dictionary) of names to values. The last two options enable assigning arbitrary values to enumerations; the others auto-assign increasing integers starting with 1 (use the start parameter to specify a different starting value). A new class derived from Enum is returned. In other words, the above assignment to Animal is equivalent to:
The reason for defaulting to 1 as the starting number and not 0 is that 0 is False in a boolean sense, but by default enum members all evaluate to True .
Pickling enums created with the functional API can be tricky as frame stack implementation details are used to try and figure out which module the enumeration is being created in (e.g. it will fail if you use a utility function in a separate module, and also may not work on IronPython or Jython). The solution is to specify the module name explicitly as follows:
If module is not supplied, and Enum cannot determine what it is, the new Enum members will not be unpicklable; to keep errors closer to the source, pickling will be disabled.
The new pickle protocol 4 also, in some circumstances, relies on __qualname__ being set to the location where pickle will be able to find the class. For example, if the class was made available in class SomeData in the global scope:
The complete signature is:
What the new enum class will record as its name.
The enum members. This can be a whitespace- or comma-separated string (values will start at 1 unless otherwise specified):
Creating Flags
Hi, again thank you for your awesome tutorial. Could you please help me with this? I don’t know what I’m doing wrong, I tryed to capture ‘this’ in the multiline string bellow with the re.finditer and the re.findall methods. Even though both methods find the word ‘this’ they gave different results. Why is this? I find the re.finditer method more informative because it gives me the span, but it is not getting the whole matches in this case.
Also, I would like to know if it is possible to pass as parameter to the re methods more than one flag. I mean, if the first word in the multiline string change to “This” I would like to pass re.I , re.M (both flags) to the re methods.
Thank you in advance.
raulfz on Jan. 30, 2021
Sorry, when I posted the previous message I haven’t finished the video, so now I know how to pass multiple flags, and even better how to apply to specific parts of my search.
raulfz on Feb. 1, 2021
Just in case someone finds the same issue I reported before, this is the problem. When I used .finditer() method I first used the re.compile() method. It’s in this re.compíle() method that I should have included the flags in order to work the same as with the findall() method.
Roy Telles on March 16, 2021
I still don’t quite understand why the fourth “SPAM” at 10:49 didn’t match. The video and transcript say it is because the “s” can be lowercase or uppercase, but then it goes on to say “literal small ‘p-a-m’” but I think it meant to say “capital ‘P-A-M’“. Considering the group contains only s’s it makes sense that the fourth doesn’t match because it would still be “sPAM” which doesn’t match ‘(s)pam’ in the regex. Thank you!
Christopher Trudeau RP Team on March 16, 2021
I believe you are talking about this snippet:
When I said “literal p-a-m”, what I mean is the literal portion of the small characters in the REGEX, not in the result. The “(?-i:s)” portion of the regex makes the “s” case insensitive, but only applies inside the brackets. This means “Spam” and “spam” match because the “pam” is small in those cases. The fourth “SPAM” doesn’t match because the “PAM” portion of the string doesn’t match the “pam” literal in the REGEX.