# Django 2.2.9.dev20191202104323 文档

# django.db.models.functions.text 源代码

    from django.db.models.expressions import Func, Value
    from django.db.models.fields import IntegerField
    from django.db.models.functions import Coalesce
    from django.db.models.lookups import Transform

    class BytesToCharFieldConversionMixin:
        """
        Convert CharField results from bytes to str.

        MySQL returns long data types (bytes) instead of chars when it can't
        determine the length of the result string. For example:
            LPAD(column1, CHAR_LENGTH(column2), ' ')
        returns the LONGTEXT (bytes) instead of VARCHAR.
        """
        def convert_value(self, value, expression, connection):
            if connection.features.db_functions_convert_bytes_to_str:
                if self.output_field.get_internal_type() == 'CharField' and isinstance(value, bytes):
                    return value.decode()
            return super().convert_value(value, expression, connection)

    class Chr(Transform):
        function = 'CHR'
        lookup_name = 'chr'

        def as_mysql(self, compiler, connection, **extra_context):
            return super().as_sql(
                compiler, connection, function='CHAR',
                template='%(function)s(%(expressions)s USING utf16)',
                **extra_context
            )

        def as_oracle(self, compiler, connection, **extra_context):
            return super().as_sql(
                compiler, connection,
                template='%(function)s(%(expressions)s USING NCHAR_CS)',
                **extra_context
            )

        def as_sqlite(self, compiler, connection, **extra_context):
            return super().as_sql(compiler, connection, function='CHAR', **extra_context)

    class ConcatPair(Func):
        """
        Concatenate two arguments together. This is used by `Concat` because not
        all backend databases support more than two arguments.
        """
        function = 'CONCAT'

        def as_sqlite(self, compiler, connection, **extra_context):
            coalesced = self.coalesce()
            return super(ConcatPair, coalesced).as_sql(
                compiler, connection, template='%(expressions)s', arg_joiner=' || ',
                **extra_context
            )

        def as_mysql(self, compiler, connection, **extra_context):
            # Use CONCAT_WS with an empty separator so that NULLs are ignored.
            return super().as_sql(
                compiler, connection, function='CONCAT_WS',
                template="%(function)s('', %(expressions)s)",
                **extra_context
            )

        def coalesce(self):
            # null on either side results in null for expression, wrap with coalesce
            c = self.copy()
            c.set_source_expressions([
                Coalesce(expression, Value('')) for expression in c.get_source_expressions()
            ])
            return c

    class Concat(Func):
        """
        Concatenate text fields together. Backends that result in an entire
        null expression when any arguments are null will wrap each argument in
        coalesce functions to ensure a non-null result.
        """
        function = None
        template = "%(expressions)s"

        def __init__(self, *expressions, **extra):
            if len(expressions) < 2:
                raise ValueError('Concat must take at least two expressions')
            paired = self._paired(expressions)
            super().__init__(paired, **extra)

        def _paired(self, expressions):
            # wrap pairs of expressions in successive concat functions
            # exp = [a, b, c, d]
            # -> ConcatPair(a, ConcatPair(b, ConcatPair(c, d))))
            if len(expressions) == 2:
                return ConcatPair(*expressions)
            return ConcatPair(expressions[0], self._paired(expressions[1:]))

    class Left(Func):
        function = 'LEFT'
        arity = 2

        def __init__(self, expression, length, **extra):
            """
            expression: the name of a field, or an expression returning a string
            length: the number of characters to return from the start of the string
            """
            if not hasattr(length, 'resolve_expression'):
                if length < 1:
                    raise ValueError("'length' must be greater than 0.")
            super().__init__(expression, length, **extra)

        def get_substr(self):
            return Substr(self.source_expressions[0], Value(1), self.source_expressions[1])

        def as_oracle(self, compiler, connection, **extra_context):
            return self.get_substr().as_oracle(compiler, connection, **extra_context)

        def as_sqlite(self, compiler, connection, **extra_context):
            return self.get_substr().as_sqlite(compiler, connection, **extra_context)

    class Length(Transform):
        """Return the number of characters in the expression."""
        function = 'LENGTH'
        lookup_name = 'length'
        output_field = IntegerField()

        def as_mysql(self, compiler, connection, **extra_context):
            return super().as_sql(compiler, connection, function='CHAR_LENGTH', **extra_context)

    class Lower(Transform):
        function = 'LOWER'
        lookup_name = 'lower'

    class LPad(BytesToCharFieldConversionMixin, Func):
        function = 'LPAD'

        def __init__(self, expression, length, fill_text=Value(' '), **extra):
            if not hasattr(length, 'resolve_expression') and length is not None and length < 0:
                raise ValueError("'length' must be greater or equal to 0.")
            super().__init__(expression, length, fill_text, **extra)

    class LTrim(Transform):
        function = 'LTRIM'
        lookup_name = 'ltrim'

    class Ord(Transform):
        function = 'ASCII'
        lookup_name = 'ord'
        output_field = IntegerField()

        def as_mysql(self, compiler, connection, **extra_context):
            return super().as_sql(compiler, connection, function='ORD', **extra_context)

        def as_sqlite(self, compiler, connection, **extra_context):
            return super().as_sql(compiler, connection, function='UNICODE', **extra_context)

    class Repeat(BytesToCharFieldConversionMixin, Func):
        function = 'REPEAT'

        def __init__(self, expression, number, **extra):
            if not hasattr(number, 'resolve_expression') and number is not None and number < 0:
                raise ValueError("'number' must be greater or equal to 0.")
            super().__init__(expression, number, **extra)

        def as_oracle(self, compiler, connection, **extra_context):
            expression, number = self.source_expressions
            length = None if number is None else Length(expression) * number
            rpad = RPad(expression, length, expression)
            return rpad.as_sql(compiler, connection, **extra_context)

    class Replace(Func):
        function = 'REPLACE'

        def __init__(self, expression, text, replacement=Value(''), **extra):
            super().__init__(expression, text, replacement, **extra)

    class Reverse(Transform):
        function = 'REVERSE'
        lookup_name = 'reverse'

        def as_oracle(self, compiler, connection, **extra_context):
            # REVERSE in Oracle is undocumented and doesn't support multi-byte
            # strings. Use a special subquery instead.
            return super().as_sql(
                compiler, connection,
                template=(
                    '(SELECT LISTAGG(s) WITHIN GROUP (ORDER BY n DESC) FROM '
                    '(SELECT LEVEL n, SUBSTR(%(expressions)s, LEVEL, 1) s '
                    'FROM DUAL CONNECT BY LEVEL <= LENGTH(%(expressions)s)) '
                    'GROUP BY %(expressions)s)'
                ),
                **extra_context
            )

    class Right(Left):
        function = 'RIGHT'

        def get_substr(self):
            return Substr(self.source_expressions[0], self.source_expressions[1] * Value(-1))

    class RPad(LPad):
        function = 'RPAD'

    class RTrim(Transform):
        function = 'RTRIM'
        lookup_name = 'rtrim'

    class StrIndex(Func):
        """
        Return a positive integer corresponding to the 1-indexed position of the
        first occurrence of a substring inside another string, or 0 if the
        substring is not found.
        """
        function = 'INSTR'
        arity = 2
        output_field = IntegerField()

        def as_postgresql(self, compiler, connection, **extra_context):
            return super().as_sql(compiler, connection, function='STRPOS', **extra_context)

    class Substr(Func):
        function = 'SUBSTRING'

        def __init__(self, expression, pos, length=None, **extra):
            """
            expression: the name of a field, or an expression returning a string
            pos: an integer > 0, or an expression returning an integer
            length: an optional number of characters to return
            """
            if not hasattr(pos, 'resolve_expression'):
                if pos < 1:
                    raise ValueError("'pos' must be greater than 0")
            expressions = [expression, pos]
            if length is not None:
                expressions.append(length)
            super().__init__(*expressions, **extra)

        def as_sqlite(self, compiler, connection, **extra_context):
            return super().as_sql(compiler, connection, function='SUBSTR', **extra_context)

        def as_oracle(self, compiler, connection, **extra_context):
            return super().as_sql(compiler, connection, function='SUBSTR', **extra_context)

    class Trim(Transform):
        function = 'TRIM'
        lookup_name = 'trim'

    class Upper(Transform):
        function = 'UPPER'
        lookup_name = 'upper'