Что такое флаг в программировании python
Flag emoji for Python.
Converts flag emoji to ASCII and other way round.
Example¶
Install¶
If you don’t see the flags in your browser, try with your phone
How it works¶
All the flag emoji are actually composed of two unicode letters. These are the 26 regional indicator symbols.
If you pair them up according to ISO 3166 some browsers and phones will display a flag. For example TW is Taiwan: �� + �� = ����
So, to encode an ASCII code like :TW: to ����, we just need to convert the ASCII T and R to the corresponding regional indicator symbols �� and ��. To reverse it, we translate the regional indicator symbols back to ASCII letters.
Functions¶
Encodes a single flag to unicode. Two letters are converted to regional indicator symbols Three or more letters/digits are converted to tag sequences. Dashes, colons and other symbols are removed from input, only a-z, A-Z and 0-9 are processed.
In general a valid flag is either a two letter code from ISO 3166 (e.g. GB ), a code from ISO 3166-2 (e.g. GBENG ) or a numeric code from ISO 3166-1. However, not all codes produce valid unicode, see http://unicode.org/reports/tr51/#flag-emoji-tag-sequences for more information. From ISO 3166-2 only England gbeng , Scotland gbsct and Wales gbwls are considered RGI (recommended for general interchange) by the Unicode Consortium, see http://www.unicode.org/Public/emoji/latest/emoji-test.txt
countrycode (str) – Two letter ISO 3166 code or a regional code from ISO 3166-2.
The unicode representation of the flag
flag. flagize ( text : str , subregions : bool = False ) → str [source] ¶
Encode flags. Replace all two letter codes :XX: with unicode flags (emoji flag sequences)
text (str) – The text
subregions (bool) – Also replace subregional/subdivision codes :xx-xxx: with unicode flags (flag emoji tag sequences).
The text with all occurrences of :XX: replaced by unicode flags
flag. dflagize ( text : str , subregions : bool = False ) → str [source] ¶
Decode flags. Replace all unicode country flags (emoji flag sequences) in text with ascii two letter code :XX:
text (str) – The text
subregions (bool) – Also replace subregional/subdivision flags (flag emoji tag sequences) with :xx-xxx:
The text with all unicode flags replaced by ascii sequence :XX:
flag. flagize_subregional ( text : str ) → str [source] ¶
Encode subregional/subdivision flags. Replace all regional codes :xx-xxx: with unicode flags (flag emoji tag sequences)
text (str) – The text
The text with all occurrences of :xx-xxx: replaced by unicode flags
flag. dflagize_subregional ( text : str ) → str [source] ¶
Decode subregional/subdivision flags. Replace all unicode regional flags (flag emoji tag sequences) in text with their ascii code :xx-xxx:
text (str) – The text
The text with all regional flags replaced by ascii sequence :xx-xxx:
class flag. Flag ( prefix_str : str = ‘:’ , suffix_str : str = ‘:’ , warn : bool = True ) [source] ¶
Use this class if you want a different prefix and suffix instead of colons. Offers the same methods as the module.
__init__ ( prefix_str : str = ‘:’ , suffix_str : str = ‘:’ , warn : bool = True ) → None [source] ¶
Set a custom prefix and suffix. Instead of :XY: it will use
To encode subregional flags, use a suffix that is either longer than 4 characters or that does not contain A-Z, a-z, 0-9 and does not start with a — (minus).
prefix_str (str) – The leading symbols
suffix_str (str) – The trailing symbols
Decode flags. Replace all unicode country flags (emoji flag sequences) in text with ascii two letter code
text (str) – The text
subregions (bool) – Also replace subregional/subdivision flags (flag emoji tag sequences) with
The text with all unicode flags replaced by ascii sequence
dflagize_subregional ( text : str ) → str [source] ¶
Decode subregional/subdivision flags. Replace all unicode regional flags (flag emoji tag sequences) in text with their ascii code
text (str) – The text
The text with all regional flags replaced by ascii sequence
flagize ( text : str , subregions : bool = False ) → str [source] ¶
Encode flags. Replace all two letter codes
For this method the suffix should not contain A-Z, a-z or 0-9 and not start with a — (minus).
text (str) – The text
subregions (bool) – Also replace subregional/subdivision codes
The text with all occurrences of
flagize_subregional ( text : str ) → str [source] ¶
Encode subregional/subdivision flags. Replace all regional codes
For this method the suffix should not contain A-Z, a-z or 0-9 and not start with a — (minus).
text (str) – The text
The text with all occurrences of
Переменные-флаги
В старые времена с помощью флагов командовали войсками. Например, если флаг поднят — нужно атаковать, опущен — отступать.
В программировании “флагом” называют переменную булевого типа, т.е. в которой хранится либо True , либо False . Их так называют потому, что управление с их помощью похоже на флаги: есть всего 2 варианта, флаг или поднят, или опущен.
Например, вам интересно, есть ли человек на видео. Вы пишете функцию, которая проходит по кадрам видео и проверяет каждый из них. Если найдёт человека, то сообщит об этом:
Обратите внимание, что True и False написаны без кавычек! Так и задумано — это не строки, а отдельный тип данных.
По умолчанию считаем, что человека нет, поэтому создаём флаг human_found = False . Далее для каждого кадра в видео проверяем, есть ли на нём человек — вызываем метод объекта frame.has_human() . Если человек найден, то меняем флаг на True . В конце выводим флаг с помощью print .
Если ни на одном кадре человек не найдётся, то флаг так и останется False , потому что условие if frame.has_human() ни разу не сработает.
Если хоть на одном кадре есть человек, то при обработке этого кадра сработает условие if frame.has_human() и флаг станет True .
Как улучшить код
Теперь усовершенствуем код. Функция станет удобнее, если откажется от вывода на экран в пользу return .
Следующим шагом ускорим работу функции. Сейчас код продолжит проверку, даже если встретит человека на самом первом кадре видео. Если кадров в video будет много, то проверка займёт время. Это даёт возможность для оптимизации. Прервём проверку сразу, как только станет ясен результат:
Теперь заметно, что от переменной human_found можно избавиться, сделав код немного лаконичнее:
Но и это ещё не всё. Флаги являются таким частым приёмом, что они попали в стандартную библиотеку Python. Код выше можно записать в одну строку:
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Flags
Just like options change the default behavior of command line tools, flags are used to change aspects of RE behavior. You have already seen flags for ignoring case and changing behavior of line anchors. Flags can be applied to entire RE using the flags optional argument or to a particular portion of RE using special groups. And both of these forms can be mixed up as well. In regular expression parlance, flags are also known as modifiers.
Flags already seen will again be discussed in this chapter for completeness sake. You’ll also learn how to combine multiple flags.
re.IGNORECASE
First up, the flag to ignore case while matching alphabets. When flags argument is used, this can be specified as re.I or re.IGNORECASE constants.
re.DOTALL
Use re.S or re.DOTALL to allow the . metacharacter to match newline characters as well.
Multiple flags can be combined using the bitwise OR operator.
re.MULTILINE
As seen earlier, re.M or re.MULTILINE flag would allow the ^ and $ anchors to work line wise.
re.VERBOSE
The re.X or re.VERBOSE flag is another provision like named capture groups to help add clarity to RE definitions. This flag allows you to use literal whitespaces for aligning purposes and add comments after the # character to break down complex RE into multiple lines.
There are a few workarounds if you need to match whitespace and # characters literally. Here’s the relevant quote from documentation:
Whitespace within the pattern is ignored, except when in a character class, or when preceded by an unescaped backslash, or within tokens like *? , (?: or (?P . When a line contains a # that is not in a character class and is not preceded by an unescaped backslash, all characters from the leftmost such # through the end of the line are ignored.
Inline comments
Comments can also be added using the (?#comment) special group. This is independent of the re.X flag.
Inline flags
- (?flags:pat) will apply flags only for this portion
- (?-flags:pat) will negate flags only for this portion
- (?flags-flags:pat) will apply and negate particular flags only for this portion
- (?flags) will apply flags for the whole RE definition
- can be specified only at the start of RE definition
- if anchors are needed, they should be specified after this group
In these ways, flags can be specified precisely only where it is needed. The flags are to be given as single letter lowercase version of short form constants. For example, i for re.I , s for re.S and so on, except L for re.L or re.LOCALE (discussed in the re.ASCII section). And as can be observed from the below examples, these do not act as capture groups.
Cheatsheet and Summary
This chapter showed some of the flags that can be used to change the default behavior of RE definition. And more special groupings were covered.
Exercises
a) Remove from the first occurrence of hat to the last occurrence of it for the given input strings. Match these markers case insensitively.
b) Delete from start if it is at the beginning of a line up to the next occurrence of the end at the end of a line. Match these markers case insensitively.
- This case sensitively
- nice and cool case insensitively
d) For the given input strings, match if the string begins with Th and also contains a line that starts with There .
Flags
absl.flags defines a distributed command line system, replacing systems like getopt() , optparse , and manual argument processing. Rather than an application having to define all flags in or near main() , each Python module defines flags that are useful to it. When one Python module imports another, it gains access to the other’s flags. (This behavior is implemented by having all modules share a common, global registry object containing all the flag information.)
The Abseil flags library includes the ability to define flag types ( boolean , float , integer , list ), autogeneration of help (in both human and machine readable format) and reading arguments from a file. It also includes the ability to automatically generate manual pages from the help flags.
Flags are defined through the use of DEFINE_* functions (where the flag’s type is used to define the value).
Example Usage
Flag Types
This is a list of the DEFINE_* ’s that you can do. All flags take a name, default value, help-string, and optional ‘short’ name (one-letter name). Some flags have other arguments, which are described with the flag.
- DEFINE_string : takes any input and interprets it as a string.
- DEFINE_bool or DEFINE_boolean : typically does not take an argument: pass —myflag to set FLAGS.myflag to True , or —nomyflag to set FLAGS.myflag to False . —myflag=true and —myflag=false are also supported, but not recommended.
- DEFINE_float : takes an input and interprets it as a floating point number. This also takes optional arguments lower_bound and upper_bound ; if the number specified on the command line is out of range, it raises a FlagError .
- DEFINE_integer : takes an input and interprets it as an integer. This also takes optional arguments lower_bound and upper_bound as for floats.
- DEFINE_enum : takes a list of strings that represents legal values. If the command-line value is not in this list, it raises a flag error; otherwise, it assigns to FLAGS.flag as a string.
- DEFINE_list : Takes a comma-separated list of strings on the command line and stores them in a Python list object.
- DEFINE_spaceseplist : Takes a space-separated list of strings on the commandline and stores them in a Python list object. For example: —myspacesepflag «foo bar baz»
- DEFINE_multi_string : The same as DEFINE_string , except the flag can be specified more than once on the command line. The result is a Python list object (list of strings), even if the flag is only on the command line once.
- DEFINE_multi_integer : The same as DEFINE_integer , except the flag can be specified more than once on the command line. The result is a Python list object (list of ints), even if the flag is only on the command line once.
- DEFINE_multi_enum : The same as DEFINE_enum , except the flag can be specified more than once on the command line. The result is a Python list object (list of strings), even if the flag is only on the command line once.
Special Flags
Some flags have special meanings:
- —help : prints a list of all key flags (see below).
- —helpshort : alias for —help .
- —helpfull : prints a list of all the flags in a human-readable fashion.
- —helpxml : prints a list of all flags, in XML format. Do not parse the output of —helpfull and —helpshort . Instead, parse the output of —helpxml .
- —flagfile=filename : read flags from file filename.
- —undefok=f1,f2 : ignore unrecognized option errors for f1,f2. For boolean flags, you should use —undefok=boolflag , and —boolflag and —noboolflag will be accepted. Do not use —undefok=noboolflag .
- — : as in getopt(). This terminates flag-processing.
Implementation
DEFINE_* creates a Flag object and registers it with a FlagValues object (typically the global FlagValues FLAGS , defined in __init__.py ). The FlagValues object can scan the command line arguments and pass flag arguments to the corresponding Flag objects for value-checking and type conversion. The converted flag values are available as attributes of the FlagValues object.
Code can access a flag through a FlagValues object, for instance flags.FLAGS.myflag . Typically, the __main__ module passes the command line arguments to flags.FLAGS for parsing. For example:
At bottom, this module calls getopt() , so getopt functionality is supported, including short- and long-style flags, and the use of — to terminate flags.
Methods defined by the flag module will throw FlagsError exceptions. The exception argument will be a human-readable string.
Additional Features
Flags Validators
Validators are for you if your program:
- requires flag X to be specified,
- needs flag Y to match a regular expression, or
- requires any more general constraint to be satisfied
Each validator represents a constraint over one flag, which is enforced starting from the initial parsing of the flags and until the program terminates.
Also, lower_bound and upper_bound for numerical flags are enforced using flag validators.
Registering Validators
If you want to enforce a constraint over one flag, use
After flag values are initially parsed, and after any change to the specified flag, method checker( flag_value ) will be executed. If constraint is not satisfied, an IllegalFlagValueError exception will be raised. See register_validator ’s docstring for a detailed explanation on how to construct your own checker.
Example Usage
A Note About —flagfile
Flags may be loaded from text files in addition to being specified on the commandline.
This means that you can throw any flags you don’t feel like typing into a file, listing one flag per line. For example:
You then specify your file with the special flag —flagfile=somefile . You can recursively nest flagfile= tokens or use multiple files on the command line. Lines beginning with a single hash ‘#’ or a double slash ‘//’ are comments in your flagfile.
Any flagfile=<filename> will be interpreted as having a relative path from the current working directory rather than from the place the file was included from: myPythonScript.py —flagfile=config/somefile.cfg
If somefile.cfg includes further —flagfile= directives, these will be referenced relative to the original CWD, not from the directory the including flagfile was found in!
The caveat applies to people who are including a series of nested files in a different directory than that from which they execute. Relative path names are always from CWD (current working directory), not from the directory of the parent include flagfile.