Pythonbasic

PythonBasic

Syntax^[语法]

Identifiers and Variables^[标志符和变量]

  • The naming of identifiers is exactly the same as in C.
  • In Python, you don’t have to declare a variable before using it.
  • In Python, a variable is more like a reference and one variable can refer to any data type.

The following assignments of variables are all valid.

a = 1.2
a = "Daisy"

a, b = "Hello", "World"
# a is "Hello" and b is "World"

a = b = [1,2,3]
# a and b are both [1,2,3]

Quotation^[引用]

  • You can enclose strings using single (‘), double (“) or triple (‘’’) quotes.
  • There is no difference between single and double quotes.
    • Python 中单引号 ‘ 和双引号 “ 使用完全相同。
  • Triple quotes can denote a string across multiple lines.
    • 使用三引号(‘’’ 或 “”“)可以指定一个多行字符串。

Reserved Words^[保留字]

Python has a smaller set of keywords than C does. All the python keywords, as shown below, contain only lowercase letters.

and, exec, not, as, finally, or, assert,
for, pass, break, from, print, class, global,
raise, continue, if, return, def, import, try,
del, in, while, elif, is, with, else,
lambda, yield, except

Lines and Indentation^[缩进]

In Python, end of a statement is marked by a newline character (no semicolon!). But we can write a single statement over multiple lines with the line continuation character (\). For example:

# use "\" to divide one statement over multiple lines
a = 1 + 2 + 3 + \
    4 + 5 + 6 + \
    7 + 8 + 9

Further, line continuation is implied inside parentheses (()), brackets ([]) and braces ({}). In the following example, the line continuation character can be omitted.

print("Hello",
      "World")
gender = ["female",
         "male"]

If you want to put multiple statements in a single line, seperate them using semicolons (;).

a = 1; b = 2; c = 3

Python does not use braces ({}) to indicate blocks of code. Code blocks are denoted by line indentation and this is rigidly enforced. Wrong indentation will cause error!

A code block starts with indentation and ends with the first unindented line. Generally four whitespaces are used for indentation. In a Jupyter notebook, you may press “tab” to input four whitespaces.

Basic IO

Till now, we have used the print() function for output for plenty of times. For input, we have the input() function, and its syntax is:

input([prompt])

prompt is a string which can be printed on the screen. This parameter is optional. Below is an example.

name = input("What is your name? ")
print(name, "is a good name :)")

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