python基础语法

编码默认为utf-8，也可以指定编码：

# -*- coding: utf-8 -*-

 

标识符是用户编程时使用的名字，用于给变量、常量、函数、语句块等命名。在python中，标识符必须以字母或下划线开头，其他部分由字母、数字和下划线组成，对大小写敏感

 

保留字即关键字，我们不能把它们用作任何标识符名称。Python 的标准库提供了一个 keyword 模块，可以输出当前版本的所有关键字：

import keyword
print(keyword.kwlist)

结果为：

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

 

单行注释：

# 大湘菜的注释

可以在空白行输入快捷键Ctrl+/来创建注释，如果在非空白行输入此快捷键会将此行代码变作注释，再次输入此快捷键又会将注释变作代码：

# print('大湘菜')

多行注释：

'''
大
湘
菜
'''

或：

"""
大
湘
菜
"""

 

python最具特色的就是使用缩进来表示代码块：

if True:
    print ("大湘菜")
else:
    print ("小湘菜")

如果缩进不一致，会报错：

IndentationError: expected an indented block

 

数字类型：

print(int(1.23))  # 整数
print(bool(1))  # 布尔值
print(float(1.23))  # 浮点数
print(complex(1))  # 复数

结果为：

1
True
1.23
(1+0j)

 

字符串

''和""完全相同：

print('微湘菜')
print("小湘菜")
print('''
大
湘
菜
''')
print("""
巨
湘
菜
""")

结果为：

微湘菜
小湘菜

大
湘
菜


巨
湘
菜

 

转义符为\，在字符串前面使用r可以让\不发生转义：

print('大\n湘\n菜')
print(r'大\n湘\n菜')

结果为：

大
湘
菜
大\n湘\n菜

 

字符串相关的运算符：

print('小湘菜' + '大湘菜')
print('大湘菜' * 2)

结果为：

小湘菜大湘菜
大湘菜大湘菜

 

字符串截取：

s = '小湘菜大湘菜'
print(s[1:3:1])  # 变量[头下标:尾下标:步长]
print(s[-1:-3:-1])  # 从右往左以-1开始

结果为：

湘菜
菜湘

 

字符串不可改变：

s = '大湘菜'
s[0] = '小'
print(s)

结果会报错：

TypeError: 'str' object does not support item assignment

 

让用户输入：

s = input('输入：')

s即为用户输入的字符串变量

 

代码组为缩进相同的一组语句构成的代码块

 

用print()方法输出，默认换行，如若要实现不换行：

print('小湘菜', end='')
print('大湘菜', end='')

结果为：

小湘菜大湘菜

 

导入模块：

import os  # 导入整个模块
from os import open  # 导入某个函数
from os import open, close  # 导入多个函数
from os import *  # 导入所有函数

 

在命令行窗口输入：

python -h

可以查看某些参数：

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     : turn on parser debugging output (for experts only, only works on
         debug builds); 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. The following options are available:

         -X faulthandler: enable faulthandler
         -X oldparser: enable the traditional LL(1) parser; also PYTHONOLDPARSER
         -X showrefcount: output the total reference count and number of used
             memory blocks when the program finishes or after each statement in the
             interactive interpreter. This only works on debug builds
         -X tracemalloc: start tracing Python memory allocations using the
             tracemalloc module. By default, only the most recent frame is stored in a
             traceback of a trace. Use -X tracemalloc=NFRAME to start tracing with a
             traceback limit of NFRAME frames
         -X importtime: show how long each import takes. It shows module name,
             cumulative time (including nested imports) and self time (excluding
             nested imports). Note that its output may be broken in multi-threaded
             application. Typical usage is python3 -X importtime -c 'import asyncio'
         -X dev: enable CPython's "development mode", introducing additional runtime
             checks which are too expensive to be enabled by default. Effect of the
             developer mode:
                * Add default warning filter, as -W default
                * Install debug hooks on memory allocators: see the PyMem_SetupDebugHooks() C function
                * Enable the faulthandler module to dump the Python traceback on a crash
                * Enable asyncio debug mode
                * Set the dev_mode attribute of sys.flags to True
                * io.IOBase destructor logs close() exceptions
         -X utf8: enable UTF-8 mode for operating system interfaces, overriding the default
             locale-aware mode. -X utf8=0 explicitly disables UTF-8 mode (even when it would
             otherwise activate automatically)
         -X pycache_prefix=PATH: enable writing .pyc files to a parallel tree rooted at the
             given directory instead of to the code tree

--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}.
PYTHONPLATLIBDIR : override sys.platlibdir.
PYTHONCASEOK : ignore case in 'import' statements (Windows).
PYTHONUTF8: if set to 1, enable the UTF-8 mode.
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 and bytes 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.
PYTHONPYCACHEPREFIX: root directory for bytecode cache (pyc) files.