2. Lexical analysis
*******************

A Python program is read by a *parser*.  Input to the parser is a
stream of *tokens*, generated by the *lexical analyzer*.  This chapter
describes how the lexical analyzer breaks a file into tokens.

Python reads program text as Unicode code points; the encoding of a
source file can be given by an encoding declaration and defaults to
UTF-8, see **PEP 3120** for details.  If the source file cannot be
decoded, a "SyntaxError" is raised.


2.1. Line structure
===================

A Python program is divided into a number of *logical lines*.


2.1.1. Logical lines
--------------------

The end of a logical line is represented by the token NEWLINE.
Statements cannot cross logical line boundaries except where NEWLINE
is allowed by the syntax (e.g., between statements in compound
statements). A logical line is constructed from one or more *physical
lines* by following the explicit or implicit *line joining* rules.


2.1.2. Physical lines
---------------------

A physical line is a sequence of characters terminated by an end-of-
line sequence.  In source files and strings, any of the standard
platform line termination sequences can be used - the Unix form using
ASCII LF (linefeed), the Windows form using the ASCII sequence CR LF
(return followed by linefeed), or the old Macintosh form using the
ASCII CR (return) character.  All of these forms can be used equally,
regardless of platform. The end of input also serves as an implicit
terminator for the final physical line.

When embedding Python, source code strings should be passed to Python
APIs using the standard C conventions for newline characters (the "\n"
character, representing ASCII LF, is the line terminator).


2.1.3. Comments
---------------

A comment starts with a hash character ("#") that is not part of a
string literal, and ends at the end of the physical line.  A comment
signifies the end of the logical line unless the implicit line joining
rules are invoked. Comments are ignored by the syntax.


2.1.4. Encoding declarations
----------------------------

If a comment in the first or second line of the Python script matches
the regular expression "coding[=:]\s*([-\w.]+)", this comment is
processed as an encoding declaration; the first group of this
expression names the encoding of the source code file. The encoding
declaration must appear on a line of its own. If it is the second
line, the first line must also be a comment-only line. The recommended
forms of an encoding expression are

   # -*- coding: <encoding-name> -*-

which is recognized also by GNU Emacs, and

   # vim:fileencoding=<encoding-name>

which is recognized by Bram Moolenaar's VIM.

If no encoding declaration is found, the default encoding is UTF-8.
In addition, if the first bytes of the file are the UTF-8 byte-order
mark ("b'\xef\xbb\xbf'"), the declared file encoding is UTF-8 (this is
supported, among others, by Microsoft's **notepad**).

If an encoding is declared, the encoding name must be recognized by
Python (see Standard Encodings). The encoding is used for all lexical
analysis, including string literals, comments and identifiers.


2.1.5. Explicit line joining
----------------------------

Two or more physical lines may be joined into logical lines using
backslash characters ("\"), as follows: when a physical line ends in a
backslash that is not part of a string literal or comment, it is
joined with the following forming a single logical line, deleting the
backslash and the following end-of-line character.  For example:

   if 1900 < year < 2100 and 1 <= month <= 12 \
      and 1 <= day <= 31 and 0 <= hour < 24 \
      and 0 <= minute < 60 and 0 <= second < 60:   # Looks like a valid date
           return 1

A line ending in a backslash cannot carry a comment.  A backslash does
not continue a comment.  A backslash does not continue a token except
for string literals (i.e., tokens other than string literals cannot be
split across physical lines using a backslash).  A backslash is
illegal elsewhere on a line outside a string literal.


2.1.6. Implicit line joining
----------------------------

Expressions in parentheses, square brackets or curly braces can be
split over more than one physical line without using backslashes. For
example:

   month_names = ['Januari', 'Februari', 'Maart',      # These are the
                  'April',   'Mei',      'Juni',       # Dutch names
                  'Juli',    'Augustus', 'September',  # for the months
                  'Oktober', 'November', 'December']   # of the year

Implicitly continued lines can carry comments.  The indentation of the
continuation lines is not important.  Blank continuation lines are
allowed. There is no NEWLINE token between implicit continuation
lines.  Implicitly continued lines can also occur within triple-quoted
strings (see below); in that case they cannot carry comments.


2.1.7. Blank lines
------------------

A logical line that contains only spaces, tabs, formfeeds and possibly
a comment, is ignored (i.e., no NEWLINE token is generated).  During
interactive input of statements, handling of a blank line may differ
depending on the implementation of the read-eval-print loop.  In the
standard interactive interpreter, an entirely blank logical line (i.e.
one containing not even whitespace or a comment) terminates a multi-
line statement.


2.1.8. Indentation
------------------

Leading whitespace (spaces and tabs) at the beginning of a logical
line is used to compute the indentation level of the line, which in
turn is used to determine the grouping of statements.

Tabs are replaced (from left to right) by one to eight spaces such
that the total number of characters up to and including the
replacement is a multiple of eight (this is intended to be the same
rule as used by Unix).  The total number of spaces preceding the first
non-blank character then determines the line's indentation.
Indentation cannot be split over multiple physical lines using
backslashes; the whitespace up to the first backslash determines the
indentation.

Indentation is rejected as inconsistent if a source file mixes tabs
and spaces in a way that makes the meaning dependent on the worth of a
tab in spaces; a "TabError" is raised in that case.

**Cross-platform compatibility note:** because of the nature of text
editors on non-UNIX platforms, it is unwise to use a mixture of spaces
and tabs for the indentation in a single source file.  It should also
be noted that different platforms may explicitly limit the maximum
indentation level.

A formfeed character may be present at the start of the line; it will
be ignored for the indentation calculations above.  Formfeed
characters occurring elsewhere in the leading whitespace have an
undefined effect (for instance, they may reset the space count to
zero).

The indentation levels of consecutive lines are used to generate
INDENT and DEDENT tokens, using a stack, as follows.

Before the first line of the file is read, a single zero is pushed on
the stack; this will never be popped off again.  The numbers pushed on
the stack will always be strictly increasing from bottom to top.  At
the beginning of each logical line, the line's indentation level is
compared to the top of the stack. If it is equal, nothing happens. If
it is larger, it is pushed on the stack, and one INDENT token is
generated.  If it is smaller, it *must* be one of the numbers
occurring on the stack; all numbers on the stack that are larger are
popped off, and for each number popped off a DEDENT token is
generated.  At the end of the file, a DEDENT token is generated for
each number remaining on the stack that is larger than zero.

Here is an example of a correctly (though confusingly) indented piece
of Python code:

   def perm(l):
           # Compute the list of all permutations of l
       if len(l) <= 1:
                     return [l]
       r = []
       for i in range(len(l)):
                s = l[:i] + l[i+1:]
                p = perm(s)
                for x in p:
                 r.append(l[i:i+1] + x)
       return r

The following example shows various indentation errors:

    def perm(l):                       # error: first line indented
   for i in range(len(l)):             # error: not indented
       s = l[:i] + l[i+1:]
           p = perm(l[:i] + l[i+1:])   # error: unexpected indent
           for x in p:
                   r.append(l[i:i+1] + x)
               return r                # error: inconsistent dedent

(Actually, the first three errors are detected by the parser; only the
last error is found by the lexical analyzer --- the indentation of
"return r" does not match a level popped off the stack.)


2.1.9. Whitespace between tokens
--------------------------------

Except at the beginning of a logical line or in string literals, the
whitespace characters space, tab and formfeed can be used
interchangeably to separate tokens.  Whitespace is needed between two
tokens only if their concatenation could otherwise be interpreted as a
different token (e.g., ab is one token, but a b is two tokens).


2.2. Other tokens
=================

Besides NEWLINE, INDENT and DEDENT, the following categories of tokens
exist: *identifiers*, *keywords*, *literals*, *operators*, and
*delimiters*. Whitespace characters (other than line terminators,
discussed earlier) are not tokens, but serve to delimit tokens. Where
ambiguity exists, a token comprises the longest possible string that
forms a legal token, when read from left to right.


2.3. Identifiers and keywords
=============================

Identifiers (also referred to as *names*) are described by the
following lexical definitions.

The syntax of identifiers in Python is based on the Unicode standard
annex UAX-31, with elaboration and changes as defined below; see also
**PEP 3131** for further details.

Within the ASCII range (U+0001..U+007F), the valid characters for
identifiers are the same as in Python 2.x: the uppercase and lowercase
letters "A" through "Z", the underscore "_" and, except for the first
character, the digits "0" through "9".

Python 3.0 introduces additional characters from outside the ASCII
range (see **PEP 3131**).  For these characters, the classification
uses the version of the Unicode Character Database as included in the
"unicodedata" module.

Identifiers are unlimited in length.  Case is significant.

   identifier   ::= xid_start xid_continue*
   id_start     ::= <all characters in general categories Lu, Ll, Lt, Lm, Lo, Nl, the underscore, and characters with the Other_ID_Start property>
   id_continue  ::= <all characters in id_start, plus characters in the categories Mn, Mc, Nd, Pc and others with the Other_ID_Continue property>
   xid_start    ::= <all characters in id_start whose NFKC normalization is in "id_start xid_continue*">
   xid_continue ::= <all characters in id_continue whose NFKC normalization is in "id_continue*">

The Unicode category codes mentioned above stand for:

* *Lu* - uppercase letters

* *Ll* - lowercase letters

* *Lt* - titlecase letters

* *Lm* - modifier letters

* *Lo* - other letters

* *Nl* - letter numbers

* *Mn* - nonspacing marks

* *Mc* - spacing combining marks

* *Nd* - decimal numbers

* *Pc* - connector punctuations

* *Other_ID_Start* - explicit list of characters in PropList.txt to
  support backwards compatibility

* *Other_ID_Continue* - likewise

All identifiers are converted into the normal form NFKC while parsing;
comparison of identifiers is based on NFKC.

A non-normative HTML file listing all valid identifier characters for
Unicode 4.1 can be found at
https://www.unicode.org/Public/13.0.0/ucd/DerivedCoreProperties.txt


2.3.1. Keywords
---------------

The following identifiers are used as reserved words, or *keywords* of
the language, and cannot be used as ordinary identifiers.  They must
be spelled exactly as written here:

   False      await      else       import     pass
   None       break      except     in         raise
   True       class      finally    is         return
   and        continue   for        lambda     try
   as         def        from       nonlocal   while
   assert     del        global     not        with
   async      elif       if         or         yield


2.3.2. Soft Keywords
--------------------

New in version 3.10.

Some identifiers are only reserved under specific contexts. These are
known as *soft keywords*.  The identifiers "match", "case" and "_" can
syntactically act as keywords in contexts related to the pattern
matching statement, but this distinction is done at the parser level,
not when tokenizing.

As soft keywords, their use with pattern matching is possible while
still preserving compatibility with existing code that uses "match",
"case" and "_" as identifier names.


2.3.3. Reserved classes of identifiers
--------------------------------------

Certain classes of identifiers (besides keywords) have special
meanings.  These classes are identified by the patterns of leading and
trailing underscore characters:

"_*"
   Not imported by "from module import *".

"_"
   In a "case" pattern within a "match" statement, "_" is a soft
   keyword that denotes a wildcard.

   Separately, the interactive interpreter makes the result of the
   last evaluation available in the variable "_". (It is stored in the
   "builtins" module, alongside built-in functions like "print".)

   Elsewhere, "_" is a regular identifier. It is often used to name
   "special" items, but it is not special to Python itself.

   Note:

     The name "_" is often used in conjunction with
     internationalization; refer to the documentation for the
     "gettext" module for more information on this convention.It is
     also commonly used for unused variables.

"__*__"
   System-defined names, informally known as "dunder" names. These
   names are defined by the interpreter and its implementation
   (including the standard library). Current system names are
   discussed in the Special method names section and elsewhere. More
   will likely be defined in future versions of Python.  *Any* use of
   "__*__" names, in any context, that does not follow explicitly
   documented use, is subject to breakage without warning.

"__*"
   Class-private names.  Names in this category, when used within the
   context of a class definition, are re-written to use a mangled form
   to help avoid name clashes between "private" attributes of base and
   derived classes. See section Identifiers (Names).


2.4. Literals
=============

Literals are notations for constant values of some built-in types.


2.4.1. String and Bytes literals
--------------------------------

String literals are described by the following lexical definitions:

   stringliteral   ::= [stringprefix](shortstring | longstring)
   stringprefix    ::= "r" | "u" | "R" | "U" | "f" | "F"
                    | "fr" | "Fr" | "fR" | "FR" | "rf" | "rF" | "Rf" | "RF"
   shortstring     ::= "'" shortstringitem* "'" | '"' shortstringitem* '"'
   longstring      ::= "'''" longstringitem* "'''" | '"""' longstringitem* '"""'
   shortstringitem ::= shortstringchar | stringescapeseq
   longstringitem  ::= longstringchar | stringescapeseq
   shortstringchar ::= <any source character except "\" or newline or the quote>
   longstringchar  ::= <any source character except "\">
   stringescapeseq ::= "\" <any source character>

   bytesliteral   ::= bytesprefix(shortbytes | longbytes)
   bytesprefix    ::= "b" | "B" | "br" | "Br" | "bR" | "BR" | "rb" | "rB" | "Rb" | "RB"
   shortbytes     ::= "'" shortbytesitem* "'" | '"' shortbytesitem* '"'
   longbytes      ::= "'''" longbytesitem* "'''" | '"""' longbytesitem* '"""'
   shortbytesitem ::= shortbyteschar | bytesescapeseq
   longbytesitem  ::= longbyteschar | bytesescapeseq
   shortbyteschar ::= <any ASCII character except "\" or newline or the quote>
   longbyteschar  ::= <any ASCII character except "\">
   bytesescapeseq ::= "\" <any ASCII character>

One syntactic restriction not indicated by these productions is that
whitespace is not allowed between the "stringprefix" or "bytesprefix"
and the rest of the literal. The source character set is defined by
the encoding declaration; it is UTF-8 if no encoding declaration is
given in the source file; see section Encoding declarations.

In plain English: Both types of literals can be enclosed in matching
single quotes ("'") or double quotes (""").  They can also be enclosed
in matching groups of three single or double quotes (these are
generally referred to as *triple-quoted strings*). The backslash ("\")
character is used to give special meaning to otherwise ordinary
characters like "n", which means 'newline' when escaped ("\n"). It can
also be used to escape characters that otherwise have a special
meaning, such as newline, backslash itself, or the quote character.
See escape sequences below for examples.

Bytes literals are always prefixed with "'b'" or "'B'"; they produce
an instance of the "bytes" type instead of the "str" type.  They may
only contain ASCII characters; bytes with a numeric value of 128 or
greater must be expressed with escapes.

Both string and bytes literals may optionally be prefixed with a
letter "'r'" or "'R'"; such strings are called *raw strings* and treat
backslashes as literal characters.  As a result, in string literals,
"'\U'" and "'\u'" escapes in raw strings are not treated specially.
Given that Python 2.x's raw unicode literals behave differently than
Python 3.x's the "'ur'" syntax is not supported.

New in version 3.3: The "'rb'" prefix of raw bytes literals has been
added as a synonym of "'br'".

New in version 3.3: Support for the unicode legacy literal
("u'value'") was reintroduced to simplify the maintenance of dual
Python 2.x and 3.x codebases. See **PEP 414** for more information.

A string literal with "'f'" or "'F'" in its prefix is a *formatted
string literal*; see Formatted string literals.  The "'f'" may be
combined with "'r'", but not with "'b'" or "'u'", therefore raw
formatted strings are possible, but formatted bytes literals are not.

In triple-quoted literals, unescaped newlines and quotes are allowed
(and are retained), except that three unescaped quotes in a row
terminate the literal.  (A "quote" is the character used to open the
literal, i.e. either "'" or """.)

Unless an "'r'" or "'R'" prefix is present, escape sequences in string
and bytes literals are interpreted according to rules similar to those
used by Standard C.  The recognized escape sequences are:

+-------------------+-----------------------------------+---------+
| Escape Sequence   | Meaning                           | Notes   |
|===================|===================================|=========|
| "\"<newline>      | Backslash and newline ignored     | (1)     |
+-------------------+-----------------------------------+---------+
| "\\"              | Backslash ("\")                   |         |
+-------------------+-----------------------------------+---------+
| "\'"              | Single quote ("'")                |         |
+-------------------+-----------------------------------+---------+
| "\""              | Double quote (""")                |         |
+-------------------+-----------------------------------+---------+
| "\a"              | ASCII Bell (BEL)                  |         |
+-------------------+-----------------------------------+---------+
| "\b"              | ASCII Backspace (BS)              |         |
+-------------------+-----------------------------------+---------+
| "\f"              | ASCII Formfeed (FF)               |         |
+-------------------+-----------------------------------+---------+
| "\n"              | ASCII Linefeed (LF)               |         |
+-------------------+-----------------------------------+---------+
| "\r"              | ASCII Carriage Return (CR)        |         |
+-------------------+-----------------------------------+---------+
| "\t"              | ASCII Horizontal Tab (TAB)        |         |
+-------------------+-----------------------------------+---------+
| "\v"              | ASCII Vertical Tab (VT)           |         |
+-------------------+-----------------------------------+---------+
| "\ooo"            | Character with octal value *ooo*  | (2,4)   |
+-------------------+-----------------------------------+---------+
| "\xhh"            | Character with hex value *hh*     | (3,4)   |
+-------------------+-----------------------------------+---------+

Escape sequences only recognized in string literals are:

+-------------------+-----------------------------------+---------+
| Escape Sequence   | Meaning                           | Notes   |
|===================|===================================|=========|
| "\N{name}"        | Character named *name* in the     | (5)     |
|                   | Unicode database                  |         |
+-------------------+-----------------------------------+---------+
| "\uxxxx"          | Character with 16-bit hex value   | (6)     |
|                   | *xxxx*                            |         |
+-------------------+-----------------------------------+---------+
| "\Uxxxxxxxx"      | Character with 32-bit hex value   | (7)     |
|                   | *xxxxxxxx*                        |         |
+-------------------+-----------------------------------+---------+

Notes:

1. A backslash can be added at the end of a line to ignore the
   newline:

      >>> 'This string will not include \
      ... backslashes or newline characters.'
      'This string will not include backslashes or newline characters.'

   The same result can be achieved using triple-quoted strings, or
   parentheses and string literal concatenation.

2. As in Standard C, up to three octal digits are accepted.

3. Unlike in Standard C, exactly two hex digits are required.

4. In a bytes literal, hexadecimal and octal escapes denote the byte
   with the given value. In a string literal, these escapes denote a
   Unicode character with the given value.

5. Changed in version 3.3: Support for name aliases [1] has been
   added.

6. Exactly four hex digits are required.

7. Any Unicode character can be encoded this way.  Exactly eight hex
   digits are required.

Unlike Standard C, all unrecognized escape sequences are left in the
string unchanged, i.e., *the backslash is left in the result*.  (This
behavior is useful when debugging: if an escape sequence is mistyped,
the resulting output is more easily recognized as broken.)  It is also
important to note that the escape sequences only recognized in string
literals fall into the category of unrecognized escapes for bytes
literals.

   Changed in version 3.6: Unrecognized escape sequences produce a
   "DeprecationWarning".  In a future Python version they will be a
   "SyntaxWarning" and eventually a "SyntaxError".

Even in a raw literal, quotes can be escaped with a backslash, but the
backslash remains in the result; for example, "r"\""" is a valid
string literal consisting of two characters: a backslash and a double
quote; "r"\"" is not a valid string literal (even a raw string cannot
end in an odd number of backslashes).  Specifically, *a raw literal
cannot end in a single backslash* (since the backslash would escape
the following quote character).  Note also that a single backslash
followed by a newline is interpreted as those two characters as part
of the literal, *not* as a line continuation.


2.4.2. String literal concatenation
-----------------------------------

Multiple adjacent string or bytes literals (delimited by whitespace),
possibly using different quoting conventions, are allowed, and their
meaning is the same as their concatenation.  Thus, ""hello" 'world'"
is equivalent to ""helloworld"".  This feature can be used to reduce
the number of backslashes needed, to split long strings conveniently
across long lines, or even to add comments to parts of strings, for
example:

   re.compile("[A-Za-z_]"       # letter or underscore
              "[A-Za-z0-9_]*"   # letter, digit or underscore
             )

Note that this feature is defined at the syntactical level, but
implemented at compile time.  The '+' operator must be used to
concatenate string expressions at run time.  Also note that literal
concatenation can use different quoting styles for each component
(even mixing raw strings and triple quoted strings), and formatted
string literals may be concatenated with plain string literals.


2.4.3. Formatted string literals
--------------------------------

New in version 3.6.

A *formatted string literal* or *f-string* is a string literal that is
prefixed with "'f'" or "'F'".  These strings may contain replacement
fields, which are expressions delimited by curly braces "{}". While
other string literals always have a constant value, formatted strings
are really expressions evaluated at run time.

Escape sequences are decoded like in ordinary string literals (except
when a literal is also marked as a raw string).  After decoding, the
grammar for the contents of the string is:

   f_string          ::= (literal_char | "{{" | "}}" | replacement_field)*
   replacement_field ::= "{" f_expression ["="] ["!" conversion] [":" format_spec] "}"
   f_expression      ::= (conditional_expression | "*" or_expr)
                      ("," conditional_expression | "," "*" or_expr)* [","]
                    | yield_expression
   conversion        ::= "s" | "r" | "a"
   format_spec       ::= (literal_char | NULL | replacement_field)*
   literal_char      ::= <any code point except "{", "}" or NULL>

The parts of the string outside curly braces are treated literally,
except that any doubled curly braces "'{{'" or "'}}'" are replaced
with the corresponding single curly brace.  A single opening curly
bracket "'{'" marks a replacement field, which starts with a Python
expression. To display both the expression text and its value after
evaluation, (useful in debugging), an equal sign "'='" may be added
after the expression. A conversion field, introduced by an exclamation
point "'!'" may follow.  A format specifier may also be appended,
introduced by a colon "':'". A replacement field ends with a closing
curly bracket "'}'".

Expressions in formatted string literals are treated like regular
Python expressions surrounded by parentheses, with a few exceptions.
An empty expression is not allowed, and both "lambda"  and assignment
expressions ":=" must be surrounded by explicit parentheses.
Replacement expressions can contain line breaks (e.g. in triple-quoted
strings), but they cannot contain comments.  Each expression is
evaluated in the context where the formatted string literal appears,
in order from left to right.

Changed in version 3.7: Prior to Python 3.7, an "await" expression and
comprehensions containing an "async for" clause were illegal in the
expressions in formatted string literals due to a problem with the
implementation.

When the equal sign "'='" is provided, the output will have the
expression text, the "'='" and the evaluated value. Spaces after the
opening brace "'{'", within the expression and after the "'='" are all
retained in the output. By default, the "'='" causes the "repr()" of
the expression to be provided, unless there is a format specified.
When a format is specified it defaults to the "str()" of the
expression unless a conversion "'!r'" is declared.

New in version 3.8: The equal sign "'='".

If a conversion is specified, the result of evaluating the expression
is converted before formatting.  Conversion "'!s'" calls "str()" on
the result, "'!r'" calls "repr()", and "'!a'" calls "ascii()".

The result is then formatted using the "format()" protocol.  The
format specifier is passed to the "__format__()" method of the
expression or conversion result.  An empty string is passed when the
format specifier is omitted.  The formatted result is then included in
the final value of the whole string.

Top-level format specifiers may include nested replacement fields.
These nested fields may include their own conversion fields and format
specifiers, but may not include more deeply nested replacement fields.
The format specifier mini-language is the same as that used by the
"str.format()" method.

Formatted string literals may be concatenated, but replacement fields
cannot be split across literals.

Some examples of formatted string literals:

   >>> name = "Fred"
   >>> f"He said his name is {name!r}."
   "He said his name is 'Fred'."
   >>> f"He said his name is {repr(name)}."  # repr() is equivalent to !r
   "He said his name is 'Fred'."
   >>> width = 10
   >>> precision = 4
   >>> value = decimal.Decimal("12.34567")
   >>> f"result: {value:{width}.{precision}}"  # nested fields
   'result:      12.35'
   >>> today = datetime(year=2017, month=1, day=27)
   >>> f"{today:%B %d, %Y}"  # using date format specifier
   'January 27, 2017'
   >>> f"{today=:%B %d, %Y}" # using date format specifier and debugging
   'today=January 27, 2017'
   >>> number = 1024
   >>> f"{number:#0x}"  # using integer format specifier
   '0x400'
   >>> foo = "bar"
   >>> f"{ foo = }" # preserves whitespace
   " foo = 'bar'"
   >>> line = "The mill's closed"
   >>> f"{line = }"
   'line = "The mill\'s closed"'
   >>> f"{line = :20}"
   "line = The mill's closed   "
   >>> f"{line = !r:20}"
   'line = "The mill\'s closed" '

A consequence of sharing the same syntax as regular string literals is
that characters in the replacement fields must not conflict with the
quoting used in the outer formatted string literal:

   f"abc {a["x"]} def"    # error: outer string literal ended prematurely
   f"abc {a['x']} def"    # workaround: use different quoting

Backslashes are not allowed in format expressions and will raise an
error:

   f"newline: {ord('\n')}"  # raises SyntaxError

To include a value in which a backslash escape is required, create a
temporary variable.

>>> newline = ord('\n')
>>> f"newline: {newline}"
'newline: 10'

Formatted string literals cannot be used as docstrings, even if they
do not include expressions.

   >>> def foo():
   ...     f"Not a docstring"
   ...
   >>> foo.__doc__ is None
   True

See also **PEP 498** for the proposal that added formatted string
literals, and "str.format()", which uses a related format string
mechanism.


2.4.4. Numeric literals
-----------------------

There are three types of numeric literals: integers, floating point
numbers, and imaginary numbers.  There are no complex literals
(complex numbers can be formed by adding a real number and an
imaginary number).

Note that numeric literals do not include a sign; a phrase like "-1"
is actually an expression composed of the unary operator '"-"' and the
literal "1".


2.4.5. Integer literals
-----------------------

Integer literals are described by the following lexical definitions:

   integer      ::= decinteger | bininteger | octinteger | hexinteger
   decinteger   ::= nonzerodigit (["_"] digit)* | "0"+ (["_"] "0")*
   bininteger   ::= "0" ("b" | "B") (["_"] bindigit)+
   octinteger   ::= "0" ("o" | "O") (["_"] octdigit)+
   hexinteger   ::= "0" ("x" | "X") (["_"] hexdigit)+
   nonzerodigit ::= "1"..."9"
   digit        ::= "0"..."9"
   bindigit     ::= "0" | "1"
   octdigit     ::= "0"..."7"
   hexdigit     ::= digit | "a"..."f" | "A"..."F"

There is no limit for the length of integer literals apart from what
can be stored in available memory.

Underscores are ignored for determining the numeric value of the
literal.  They can be used to group digits for enhanced readability.
One underscore can occur between digits, and after base specifiers
like "0x".

Note that leading zeros in a non-zero decimal number are not allowed.
This is for disambiguation with C-style octal literals, which Python
used before version 3.0.

Some examples of integer literals:

   7     2147483647                        0o177    0b100110111
   3     79228162514264337593543950336     0o377    0xdeadbeef
         100_000_000_000                   0b_1110_0101

Changed in version 3.6: Underscores are now allowed for grouping
purposes in literals.


2.4.6. Floating point literals
------------------------------

Floating point literals are described by the following lexical
definitions:

   floatnumber   ::= pointfloat | exponentfloat
   pointfloat    ::= [digitpart] fraction | digitpart "."
   exponentfloat ::= (digitpart | pointfloat) exponent
   digitpart     ::= digit (["_"] digit)*
   fraction      ::= "." digitpart
   exponent      ::= ("e" | "E") ["+" | "-"] digitpart

Note that the integer and exponent parts are always interpreted using
radix 10. For example, "077e010" is legal, and denotes the same number
as "77e10". The allowed range of floating point literals is
implementation-dependent.  As in integer literals, underscores are
supported for digit grouping.

Some examples of floating point literals:

   3.14    10.    .001    1e100    3.14e-10    0e0    3.14_15_93

Changed in version 3.6: Underscores are now allowed for grouping
purposes in literals.


2.4.7. Imaginary literals
-------------------------

Imaginary literals are described by the following lexical definitions:

   imagnumber ::= (floatnumber | digitpart) ("j" | "J")

An imaginary literal yields a complex number with a real part of 0.0.
Complex numbers are represented as a pair of floating point numbers
and have the same restrictions on their range.  To create a complex
number with a nonzero real part, add a floating point number to it,
e.g., "(3+4j)".  Some examples of imaginary literals:

   3.14j   10.j    10j     .001j   1e100j   3.14e-10j   3.14_15_93j


2.5. Operators
==============

The following tokens are operators:

   +       -       *       **      /       //      %      @
   <<      >>      &       |       ^       ~       :=
   <       >       <=      >=      ==      !=


2.6. Delimiters
===============

The following tokens serve as delimiters in the grammar:

   (       )       [       ]       {       }
   ,       :       .       ;       @       =       ->
   +=      -=      *=      /=      //=     %=      @=
   &=      |=      ^=      >>=     <<=     **=

The period can also occur in floating-point and imaginary literals.  A
sequence of three periods has a special meaning as an ellipsis
literal. The second half of the list, the augmented assignment
operators, serve lexically as delimiters, but also perform an
operation.

The following printing ASCII characters have special meaning as part
of other tokens or are otherwise significant to the lexical analyzer:

   '       "       #       \

The following printing ASCII characters are not used in Python.  Their
occurrence outside string literals and comments is an unconditional
error:

   $       ?       `

-[ Footnotes ]-

[1] https://www.unicode.org/Public/11.0.0/ucd/NameAliases.txt
