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Python 学习笔记 2. 基础语法 ​

🏷️ Python Python 学习笔记

保留字符 ​

andexecnot
assertfinallyor
breakforpass
classfromprint
continueglobalraise
defifreturn
delimporttry
elifinwhile
elseiswith
exceptlambdayield

行和缩进 ​

Python 最具特色的就是用缩进来写模块。

缩进的空白数量是可变的，但是所有代码块语句必须包含相同的缩进空白数量，这个必须严格执行。

以下示例缩进为四个空格:

python
>>> if True:
...     print ("True")
... else:
...     print ("False")
...
True
>>>

建议你在每个缩进层次使用 单个制表符 或 两个空格 或 四个空格 , 切记不能混用。

注释 ​

单行注释采用 # 开头。

python
# 第一个注释
print ("Hello, Python!") # 第二个注释

多行注释使用三个单引号 ( ''' ) 或三个双引号 ( """ )。

python
'''
这是多行注释，使用单引号。
这是多行注释，使用单引号。
这是多行注释，使用单引号。
'''

"""
这是多行注释，使用双引号。
这是多行注释，使用双引号。
这是多行注释，使用双引号。
"""

多行语句 ​

可以使用斜杠（ \ ）将一行的语句分为多行显示。

python
>>> item_one = 'one '
>>> item_two = 'two '
>>> item_three = 'three '
>>> total = item_one + \
...         item_two + \
...         item_three
>>> total
'one two three '
>>>

命令行参数 ​

使用 -h 参数查看各参数帮助信息。

下面是 python 3.7.3 执行 python -h 的结果：

usage: python [option] ... [-c cmd | -m mod | file | -] [arg] ...
Options and arguments (and corresponding environment variables):

  • -b : issue warnings about str(bytes_instance), str(bytearray_instance) and comparing bytes/bytearray with str. (-bb: issue errors)
  • -B : don't write .pyc files on import; also PYTHONDONTWRITEBYTECODE=x
  • -c cmd : program passed in as string (terminates option list)
  • -d : debug output from parser; also PYTHONDEBUG=x
  • -E : ignore PYTHON* environment variables (such as PYTHONPATH)
  • -h : print this help message and exit (also --help)
  • -i : inspect interactively after running script; forces a prompt even if stdin does not appear to be a terminal; also PYTHONINSPECT=x
  • -I : isolate Python from the user's environment (implies -E and -s)
  • -m mod : run library module as a script (terminates option list)
  • -O : remove assert and __debug__-dependent statements; add .opt-1 before .pyc extension; also PYTHONOPTIMIZE=x
  • -OO : do -O changes and also discard docstrings; add .opt-2 before .pyc extension
  • -q : don't print version and copyright messages on interactive startup
  • -s : don't add user site directory to sys.path; also PYTHONNOUSERSITE
  • -S : don't imply 'import site' on initialization
  • -u : force the stdout and stderr streams to be unbuffered; this option has no effect on stdin; also PYTHONUNBUFFERED=x
  • -v : verbose (trace import statements); also PYTHONVERBOSE=x can be supplied multiple times to increase verbosity
  • -V : print the Python version number and exit (also --version) when given twice, print more information about the build
  • -W arg : warning control; arg is action:message:category:module:lineno also PYTHONWARNINGS=arg
  • -x : skip first line of source, allowing use of non-Unix forms of #!cmd
  • -X opt : set implementation-specific option
  • --check-hash-based-pycs always|default|never: control how Python invalidates hash-based .pyc files
  • file : program read from script file
  • - : program read from stdin (default; interactive mode if a tty)
  • arg ...: arguments passed to program in sys.argv[1:]

Other environment variables:

  • PYTHONSTARTUP: file executed on interactive startup (no default)
  • PYTHONPATH : ';'-separated list of directories prefixed to the default module search path. The result is sys.path.
  • PYTHONHOME : alternate <prefix> directory (or <prefix>;<exec_prefix>). The default module search path uses <prefix>\python{major}{minor}.
  • PYTHONCASEOK : ignore case in 'import' statements (Windows).
  • PYTHONIOENCODING: Encoding[:errors] used for stdin/stdout/stderr.
  • PYTHONFAULTHANDLER: dump the Python traceback on fatal errors.
  • PYTHONHASHSEED: if this variable is set to 'random', a random value is used to seed the hashes of str, bytes and datetime objects. It can also be set to an integer in the range [0,4294967295] to get hash values with a predictable seed.
  • PYTHONMALLOC: set the Python memory allocators and/or install debug hooks on Python memory allocators. Use PYTHONMALLOC=debug to install debug hooks.
  • PYTHONCOERCECLOCALE: if this variable is set to 0, it disables the locale coercion behavior. Use PYTHONCOERCECLOCALE=warn to request display of locale coercion and locale compatibility warnings on stderr.
  • PYTHONBREAKPOINT: if this variable is set to 0, it disables the default debugger. It can be set to the callable of your debugger of choice.
  • PYTHONDEVMODE: enable the development mode.