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 one the following end-of-line sequences:

• the Unix form using ASCII LF (linefeed),

• the Windows form using the ASCII sequence CR LF (return followed by linefeed),

• the ‘Classic Mac OS’ form using the ASCII CR (return) character.

Regardless of platform, each of these sequences is replaced by a single ASCII LF (linefeed) character. (This is done even inside string literals.) Each line can use any of the sequences; they do not need to be consistent within a file.

The end of input also serves as an implicit terminator for the final physical line.

Formally:

newline: <ASCII LF> | <ASCII CR> <ASCII LF> | <ASCII CR>

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. If the implicit or explicit encoding of a file is UTF-8, an initial UTF-8 byte-order mark (b'\xef\xbb\xbf') is ignored rather than being a syntax error.

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.

All lexical analysis, including string literals, comments and identifiers, works on Unicode text decoded using the source encoding. Any Unicode code point, except the NUL control character, can appear in Python source.

source_character:  <any Unicode code point, except NUL>

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 (that is, 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:  ' ' | tab | formfeed

Whitespace is needed between two tokens only if their concatenation could otherwise be interpreted as a different token. For example, ab is one token, but a b is two tokens. However, +a and + a both produce two tokens, + and a, as +a is not a valid token.

2.1.10. End marker¶

At the end of non-interactive input, the lexical analyzer generates an ENDMARKER token.

2.3.1. Keywords¶

The following names 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¶

Some names are only reserved under specific contexts. These are known as soft keywords:

• match, case, and _, when used in the match statement.

• type, when used in the type statement.

• lazy, when used before an import statement.

These syntactically act as keywords in their specific contexts, but this distinction is done at the parser level, not when tokenizing.

As soft keywords, their use in the grammar is possible while still preserving compatibility with existing code that uses these names 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).

2.3.4. Non-ASCII characters in names¶

Names that contain non-ASCII characters need additional normalization and validation beyond the rules and grammar explained above. For example, ř_1, 蛇, or साँप are valid names, but r〰2, €, or 🐍 are not.

This section explains the exact rules.

All names are converted into the normalization form NFKC while parsing. This means that, for example, some typographic variants of characters are converted to their “basic” form. For example, fiⁿₐˡᵢᶻₐᵗᵢᵒₙ normalizes to finalization, so Python treats them as the same name:

>>> fiⁿₐˡᵢᶻₐᵗᵢᵒₙ = 3
>>> finalization
3

Similarly to how ASCII-only names must contain only letters, digits and the underscore, and cannot start with a digit, a valid name must start with a character in the “letter-like” set xid_start, and the remaining characters must be in the “letter- and digit-like” set xid_continue.

These sets are based on the XID_Start and XID_Continue sets as defined by the Unicode standard annex UAX-31. Python’s xid_start additionally includes the underscore (_). Note that Python does not necessarily conform to UAX-31.

A non-normative listing of characters in the XID_Start and XID_Continue sets as defined by Unicode is available in the DerivedCoreProperties.txt file in the Unicode Character Database. For reference, the construction rules for the xid_* sets are given below.

The set id_start is defined as the union of:

• Unicode category <Lu> - uppercase letters (includes A to Z)

• Unicode category <Ll> - lowercase letters (includes a to z)

• Unicode category <Lt> - titlecase letters

• Unicode category <Lm> - modifier letters

• Unicode category <Lo> - other letters

• Unicode category <Nl> - letter numbers

• {"_"} - the underscore

• <Other_ID_Start> - an explicit set of characters in PropList.txt to support backwards compatibility

The set xid_start then closes this set under NFKC normalization, by removing all characters whose normalization is not of the form id_start id_continue*.

The set id_continue is defined as the union of:

• id_start (see above)

• Unicode category <Nd> - decimal numbers (includes 0 to 9)

• Unicode category <Pc> - connector punctuations

• Unicode category <Mn> - nonspacing marks

• Unicode category <Mc> - spacing combining marks

• <Other_ID_Continue> - another explicit set of characters in PropList.txt to support backwards compatibility

Again, xid_continue closes this set under NFKC normalization.

Unicode categories use the version of the Unicode Character Database as included in the unicodedata module.

2.5.1. Triple-quoted strings¶

Strings can also be enclosed in matching groups of three single or double quotes. These are generally referred to as triple-quoted strings:

"""This is a triple-quoted string."""

In triple-quoted literals, unescaped quotes are allowed (and are retained), except that three unescaped quotes in a row terminate the literal, if they are of the same kind (' or ") used at the start:

"""This string has "quotes" inside."""

Unescaped newlines are also allowed and retained:

'''This triple-quoted string
continues on the next line.'''

2.5.2. String prefixes¶

String literals can have an optional prefix that influences how the content of the literal is parsed, for example:

b"data"
f'{result=}'

The allowed prefixes are:

• b: Bytes literal

• r: Raw string

• f: Formatted string literal (“f-string”)

• t: Template string literal (“t-string”)

• u: No effect (allowed for backwards compatibility)

See the linked sections for details on each type.

Prefixes are case-insensitive (for example, ‘B’ works the same as ‘b’). The ‘r’ prefix can be combined with ‘f’, ‘t’ or ‘b’, so ‘fr’, ‘rf’, ‘tr’, ‘rt’, ‘br’, and ‘rb’ are also valid prefixes.

2.5.3. Formal grammar¶

String literals, except “f-strings” and “t-strings”, are described by the following lexical definitions.

These definitions use negative lookaheads (!) to indicate that an ending quote ends the literal.

STRING:          [stringprefix] (stringcontent)
stringprefix:    <("r" | "u" | "b" | "br" | "rb"), case-insensitive>
stringcontent:
   | "'''" ( !"'''" longstringitem)* "'''"
   | '"""' ( !'"""' longstringitem)* '"""'
   | "'" ( !"'" stringitem)* "'"
   | '"' ( !'"' stringitem)* '"'
stringitem:      stringchar | stringescapeseq
stringchar:      <any source_character, except backslash and newline>
longstringitem:  stringitem | newline
stringescapeseq: "\" <any source_character>

Note that as in all lexical definitions, whitespace is significant. In particular, the prefix (if any) must be immediately followed by the starting quote.

2.5.4. Escape sequences¶

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:

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

>>> print('C:\\Program Files')
C:\Program Files

>>> print('\' and \"')
' and "

>>> '\120'
'P'

>>> '\x50'
'P'

>>> '\N{LATIN CAPITAL LETTER P}'
'P'
>>> '\N{SNAKE}'
'🐍'

>>> '\u1234'
'ሴ'
>>> '\U0001f40d'
'🐍'

>>> print('\q')
\q
>>> list('\q')
['\\', 'q']

2.5.5. Bytes literals¶

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 escape sequences (typically Hexadecimal character or Octal character):

>>> b'\x89PNG\r\n\x1a\n'
b'\x89PNG\r\n\x1a\n'
>>> list(b'\x89PNG\r\n\x1a\n')
[137, 80, 78, 71, 13, 10, 26, 10]

Similarly, a zero byte must be expressed using an escape sequence (typically \0 or \x00).

2.5.6. Raw string literals¶

Both string and bytes literals may optionally be prefixed with a letter ‘r’ or ‘R’; such constructs are called raw string literals and raw bytes literals respectively and treat backslashes as literal characters. As a result, in raw string literals, escape sequences are not treated specially:

>>> r'\d{4}-\d{2}-\d{2}'
'\\d{4}-\\d{2}-\\d{2}'

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.

2.5.7. f-strings¶

A formatted string literal or f-string is a string literal that is prefixed with ‘f’ or ‘F’. Unlike other string literals, f-strings do not have a constant value. They may contain replacement fields delimited by curly braces {}. Replacement fields contain expressions which are evaluated at run time. For example:

>>> who = 'nobody'
>>> nationality = 'Spanish'
>>> f'{who.title()} expects the {nationality} Inquisition!'
'Nobody expects the Spanish Inquisition!'

Any doubled curly braces ({{ or }}) outside replacement fields are replaced with the corresponding single curly brace:

>>> print(f'{{...}}')
{...}

Other characters outside replacement fields are treated like in ordinary string literals. This means that escape sequences are decoded (except when a literal is also marked as a raw string), and newlines are possible in triple-quoted f-strings:

>>> name = 'Galahad'
>>> favorite_color = 'blue'
>>> print(f'{name}:\t{favorite_color}')
Galahad:       blue
>>> print(rf"C:\Users\{name}")
C:\Users\Galahad
>>> print(f'''Three shall be the number of the counting
... and the number of the counting shall be three.''')
Three shall be the number of the counting
and the number of the counting shall be three.

Expressions in formatted string literals are treated like regular Python expressions. Each expression is evaluated in the context where the formatted string literal appears, in order from left to right. An empty expression is not allowed, and both lambda and assignment expressions := must be surrounded by explicit parentheses:

>>> f'{(half := 1/2)}, {half * 42}'
'0.5, 21.0'

Reusing the outer f-string quoting type inside a replacement field is permitted:

>>> a = dict(x=2)
>>> f"abc {a["x"]} def"
'abc 2 def'

Backslashes are also allowed in replacement fields and are evaluated the same way as in any other context:

>>> a = ["a", "b", "c"]
>>> print(f"List a contains:\n{"\n".join(a)}")
List a contains:
a
b
c

It is possible to nest f-strings:

>>> name = 'world'
>>> f'Repeated:{f' hello {name}' * 3}'
'Repeated: hello world hello world hello world'

Portable Python programs should not use more than 5 levels of nesting.

Replacement expressions can contain newlines in both single-quoted and triple-quoted f-strings and they can contain comments. Everything that comes after a # inside a replacement field is a comment (even closing braces and quotes). This means that replacement fields with comments must be closed in a different line:

>>> a = 2
>>> f"abc{a  # This comment  }"  continues until the end of the line
...       + 3}"
'abc5'

After the expression, replacement fields may optionally contain:

• a debug specifier – an equal sign (=), optionally surrounded by whitespace on one or both sides;

• a conversion specifier – !s, !r or !a; and/or

• a format specifier prefixed with a colon (:).

See the Standard Library section on f-strings for details on how these fields are evaluated.

As that section explains, format specifiers are passed as the second argument to the format() function to format a replacement field value. For example, they can be used to specify a field width and padding characters using the Format Specification Mini-Language:

>>> number = 14.3
>>> f'{number:20.7f}'
'          14.3000000'

Top-level format specifiers may include nested replacement fields:

>>> field_size = 20
>>> precision = 7
>>> f'{number:{field_size}.{precision}f}'
'          14.3000000'

These nested fields may include their own conversion fields and format specifiers:

>>> number = 3
>>> f'{number:{field_size}}'
'                   3'
>>> f'{number:{field_size:05}}'
'00000000000000000003'

However, these nested fields may not include more deeply nested replacement fields.

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

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

2.5.8. t-strings¶

A template string literal or t-string is a string literal that is prefixed with ‘t’ or ‘T’. These strings follow the same syntax rules as formatted string literals. For differences in evaluation rules, see the Standard Library section on t-strings

2.5.9. Formal grammar for f-strings¶

F-strings are handled partly by the lexical analyzer, which produces the tokens FSTRING_START, FSTRING_MIDDLE and FSTRING_END, and partly by the parser, which handles expressions in the replacement field. The exact way the work is split is a CPython implementation detail.

Correspondingly, the f-string grammar is a mix of lexical and syntactic definitions.

Whitespace is significant in these situations:

• There may be no whitespace in FSTRING_START (between the prefix and quote).

• Whitespace in FSTRING_MIDDLE is part of the literal string contents.

• In fstring_replacement_field, if f_debug_specifier is present, all whitespace after the opening brace until the f_debug_specifier, as well as whitespace immediately following f_debug_specifier, is retained as part of the expression.

  CPython implementation detail: The expression is not handled in the tokenization phase; it is retrieved from the source code using locations of the { token and the token after =.

The FSTRING_MIDDLE definition uses negative lookaheads (!) to indicate special characters (backslash, newline, {, }) and sequences (f_quote).

fstring:    FSTRING_START fstring_middle* FSTRING_END

FSTRING_START:      fstringprefix ("'" | '"' | "'''" | '"""')
FSTRING_END:        f_quote
fstringprefix:      <("f" | "fr" | "rf"), case-insensitive>
f_debug_specifier:  '='
f_quote:            <the quote character(s) used in FSTRING_START>

fstring_middle:
   | fstring_replacement_field
   | FSTRING_MIDDLE
FSTRING_MIDDLE:
   | (!"\" !newline !'{' !'}' !f_quote) source_character
   | stringescapeseq
   | "{{"
   | "}}"
   | <newline, in triple-quoted f-strings only>
fstring_replacement_field:
   | '{' f_expression [f_debug_specifier] [fstring_conversion]
         [fstring_full_format_spec] '}'
fstring_conversion:
   | "!" ("s" | "r" | "a")
fstring_full_format_spec:
   | ':' fstring_format_spec*
fstring_format_spec:
   | FSTRING_MIDDLE
   | fstring_replacement_field
f_expression:
   | ','.(conditional_expression | "*" or_expr)+ [","]
   | yield_expression

The grammar for t-strings is identical to the one for f-strings, with t instead of f at the beginning of rule and token names and in the prefix.

tstring:    TSTRING_START tstring_middle* TSTRING_END

<rest of the t-string grammar is omitted; see above>

2.6.1. Integer literals¶

Integer literals denote whole numbers. For example:

7
3
2147483647

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

7922816251426433759354395033679228162514264337593543950336

Underscores can be used to group digits for enhanced readability, and are ignored for determining the numeric value of the literal. For example, the following literals are equivalent:

100_000_000_000
100000000000
1_00_00_00_00_000

Underscores can only occur between digits. For example, _123, 321_, and 123__321 are not valid literals.

Integers can be specified in binary (base 2), octal (base 8), or hexadecimal (base 16) using the prefixes 0b, 0o and 0x, respectively. Hexadecimal digits 10 through 15 are represented by letters A-F, case-insensitive. For example:

0b100110111
0b_1110_0101
0o177
0o377
0xdeadbeef
0xDead_Beef

An underscore can follow the base specifier. For example, 0x_1f is a valid literal, but 0_x1f and 0x__1f are not.

Leading zeros in a non-zero decimal number are not allowed. For example, 0123 is not a valid literal. This is for disambiguation with C-style octal literals, which Python used before version 3.0.

Formally, integer literals are described by the following lexical definitions:

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

2.6.2. Floating-point literals¶

Floating-point (float) literals, such as 3.14 or 1.5, denote approximations of real numbers.

They consist of integer and fraction parts, each composed of decimal digits. The parts are separated by a decimal point, .:

2.71828
4.0

Unlike in integer literals, leading zeros are allowed. For example, 077.010 is legal, and denotes the same number as 77.01.

As in integer literals, single underscores may occur between digits to help readability:

96_485.332_123
3.14_15_93

Either of these parts, but not both, can be empty. For example:

10.  # (equivalent to 10.0)
.001  # (equivalent to 0.001)

Optionally, the integer and fraction may be followed by an exponent: the letter e or E, followed by an optional sign, + or -, and a number in the same format as the integer and fraction parts. The e or E represents “times ten raised to the power of”:

1.0e3  # (represents 1.0×10³, or 1000.0)
1.166e-5  # (represents 1.166×10⁻⁵, or 0.00001166)
6.02214076e+23  # (represents 6.02214076×10²³, or 602214076000000000000000.)

In floats with only integer and exponent parts, the decimal point may be omitted:

1e3  # (equivalent to 1.e3 and 1.0e3)
0e0  # (equivalent to 0.)

Formally, floating-point literals are described by the following lexical definitions:

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

2.6.3. Imaginary literals¶

Python has complex number objects, but no complex literals. Instead, imaginary literals denote complex numbers with a zero real part.

For example, in math, the complex number 3+4.2i is written as the real number 3 added to the imaginary number 4.2i. Python uses a similar syntax, except the imaginary unit is written as j rather than i:

3+4.2j

This is an expression composed of the integer literal 3, the operator ‘+’, and the imaginary literal 4.2j. Since these are three separate tokens, whitespace is allowed between them:

3 + 4.2j

No whitespace is allowed within each token. In particular, the j suffix, may not be separated from the number before it.

The number before the j has the same syntax as a floating-point literal. Thus, the following are valid imaginary literals:

4.2j
3.14j
10.j
.001j
1e100j
3.14e-10j
3.14_15_93j

Unlike in a floating-point literal the decimal point can be omitted if the imaginary number only has an integer part. The number is still evaluated as a floating-point number, not an integer:

10j
0j
1000000000000000000000000j   # equivalent to 1e+24j

The j suffix is case-insensitive. That means you can use J instead:

3.14J   # equivalent to 3.14j

Formally, imaginary literals are described by the following lexical definition:

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