我有一个很长的疑问。我想用Python将它分成几行。在JavaScript中实现这一点的一种方法是使用几个句子并用+运算符将它们连接起来(我知道,也许这不是最有效的方法,但我并不真正关心这个阶段的性能,只是代码的可读性)。例子:

var long_string = 'some text not important. just garbage to' +
                      'illustrate my example';

我尝试在Python中做类似的事情,但没有成功,所以我使用了\来拆分长字符串。然而,我不确定这是否是唯一/最好/最蟒蛇的做法。看起来很尴尬。实际代码:

query = 'SELECT action.descr as "action", '\
    'role.id as role_id,'\
    'role.descr as role'\
    'FROM '\
    'public.role_action_def,'\
    'public.role,'\
    'public.record_def, '\
    'public.action'\
    'WHERE role.id = role_action_def.role_id AND'\
    'record_def.id = role_action_def.def_id AND'\
    'action.id = role_action_def.action_id AND'\
    'role_action_def.account_id = ' + account_id + ' AND'\
    'record_def.account_id=' + account_id + ' AND'\
    'def_id=' + def_id

当前回答

我发现,当构建长字符串时,您通常会做一些类似于构建SQL查询的事情,在这种情况下,这是最好的:

query = ' '.join((  # Note double parentheses. join() takes an iterable
    "SELECT foo",
    "FROM bar",
    "WHERE baz",
))

Levon的建议很好,但可能容易出错:

query = (
    "SELECT foo"
    "FROM bar"
    "WHERE baz"
)

query == "SELECT fooFROM barWHERE baz"  # Probably not what you want

其他回答

您还可以将SQL语句放置在单独的文件action.SQL中,并使用以下命令将其加载到.py文件中:

with open('action.sql') as f:
   query = f.read()

因此,SQL语句将与Python代码分离。如果SQL语句中有需要从Python填充的参数,则可以使用字符串格式(如%s或{field})。

我个人认为,以下是用Python编写原始SQL查询的最佳(简单、安全和Python化)方法,尤其是在使用Python的sqlite3模块时:

query = '''
    SELECT
        action.descr as action,
        role.id as role_id,
        role.descr as role
    FROM
        public.role_action_def,
        public.role,
        public.record_def,
        public.action
    WHERE
        role.id = role_action_def.role_id
        AND record_def.id = role_action_def.def_id
        AND action.id = role_action_def.action_id
        AND role_action_def.account_id = ?
        AND record_def.account_id = ?
        AND def_id = ?
'''
vars = (account_id, account_id, def_id)   # a tuple of query variables
cursor.execute(query, vars)   # using Python's sqlite3 module

Pros

整洁而简单的代码(Pythonic!)避免SQL注入与Python 2和Python 3兼容(毕竟是Pythonic)不需要字符串串联无需确保每行最右边的字符是空格

Cons

由于查询中的变量被替换为?占位符,它可能会变得有点难以跟踪哪个?当查询中有很多Python变量时,将由哪个Python变量替换。

其他人已经提到了圆括号方法,但我想补充一点,使用圆括号时,允许内联注释。

对每个片段的注释:

nursery_rhyme = (
    'Mary had a little lamb,'          # Comments are great!
    'its fleece was white as snow.'
    'And everywhere that Mary went,'
    'her sheep would surely go.'       # What a pesky sheep.
)

继续后不允许注释:

使用反斜杠行延续符(\)时,不允许使用注释。您将收到一个SyntaxError:一行接一行的连续字符错误。

nursery_rhyme = 'Mary had a little lamb,' \  # These comments
    'its fleece was white as snow.'       \  # are invalid!
    'And everywhere that Mary went,'      \
    'her sheep would surely go.'
# => SyntaxError: unexpected character after line continuation character

Regex字符串的更好注释:

基于以下示例https://docs.python.org/3/library/re.html#re.VERBOSE,

a = re.compile(
    r'\d+'  # the integral part
    r'\.'   # the decimal point
    r'\d*'  # some fractional digits
)
# Using VERBOSE flag, IDE usually can't syntax highight the string comment.
a = re.compile(r"""\d +  # the integral part
                   \.    # the decimal point
                   \d *  # some fractional digits""", re.X)

你说的是多行字符串吗?简单,使用三引号开始和结束它们。

s = """ this is a very
        long string if I had the
        energy to type more and more ..."""

您也可以使用单引号(当然,在开始和结束处有3个引号),并像对待任何其他字符串一样对待生成的字符串。

注意:与任何字符串一样,起始引号和结束引号之间的任何内容都将成为字符串的一部分,因此本示例有一个前导空格(如@root45所指出的)。此字符串还将包含空格和换行符。

即。,:

' this is a very\n        long string if I had the\n        energy to type more and more ...'

最后,您还可以在Python中构造如下的长行:

 s = ("this is a very"
      "long string too"
      "for sure ..."
     )

这将不包括任何额外的空格或换行符(这是一个故意的示例,显示跳过空格的效果):

'this is a verylong string toofor sure ...'

不需要逗号,只需将要连接在一起的字符串放入一对括号中,并确保考虑到任何需要的空格和换行符。

tl;dr:使用“”和“”来换行字符串,如中所示

string = """\
This is a long string
spanning multiple lines.
"""

从Python官方文档中:

字符串文本可以跨越多行。一种方法是使用三引号:“”“…”“”或“”“”…“”。行的结束是自动的包含在字符串中,但可以通过添加\在线路的末端。以下示例:

print("""\
Usage: thingy [OPTIONS]
     -h                        Display this usage message
     -H hostname               Hostname to connect to
""")

生成以下输出(注意,初始换行符不是包括在内):

Usage: thingy [OPTIONS]
     -h                        Display this usage message
     -H hostname               Hostname to connect to