标签：python
2017.4.21
argument
A value passed to a function (or method) when calling the function. There are two kinds of argument:
Arguments are assigned to the named local variables in a function body. See the Calls section for the rules governing this assignment. Syntactically, any expression can be used to represent an argument; the evaluated value is assigned to the local variable.
keyword argument: an argument preceded by an identifier (e.g. name=) in a function call or passed as a value in a dictionary preceded by **. For example, 3 and 5 are both keyword arguments in the following calls to complex():
complex(real=3, imag=5)complex(**{‘real‘: 3, ‘imag‘: 5})positional argument: an argument that is not a keyword argument. Positional arguments can appear at the beginning of an argument list and/or be passed as elements of an iterable preceded by *. For example, 3 and 5 are both positional arguments in the following calls:
complex(3, 5)complex(*(3, 5))
By default, Python source files are treated as encoded in UTF-8. In that encoding,although the standard library only uses ASCII characters for identifiers.
To declare an encoding other than the default one, a special comment line should be added as the first line of the file. The syntax is as follows:
# -*- coding: encoding -*-
where encoding is one of the valid codecs supported by Python.
For example, to declare that Windows-1252 encoding is to be used, the first line of your source code file should be:
# -*- coding: cp-1252 -*-
One exception to the first line rule is when the source code starts with a UNIX “shebang” line. In this case, the encoding declaration should be added as the second line of the file. For example:
#!/usr/bin/env python3# -*- coding: cp-1252 -*-
Division (/) always returns a float. To do floor division and get an integer result (discarding any fractional result) you can use the // operator; to calculate the remainder you can use %:
>>> 17 / 3 # classic division returns a float5.666666666666667 >>> 17 // 3 # floor division discards the fractional part5 >>> 17 % 3 # the % operator returns the remainder of the division2 >>> 5 * 3 + 2 # result * divisor + remainder17
With Python, it is possible to use the ** operator to calculate powers [1]:
>>> 5 ** 2 # 5 squared25>>> 2 ** 7 # 2 to the power of 7128
| [1] | Since ** has higher precedence than -, -3**2 will be interpreted as -(3**2) and thus result in -9. To avoid this and get 9, you can use (-3)**2. |
浮点数和整型数值计算结果还是浮点数。
>>> 3 * 3.75 / 1.57.5>>> 7.0 / 23.5
交互式代码中，_代表上一个表达式。
>>> tax = 12.5 / 100>>> price = 100.50>>> price * tax12.5625>>> price + _113.0625>>> round(_, 2)113.06
In addition to int and float, Python supports other types of numbers, such as Decimal and Fraction. Python also has built-in support for complex numbers, and uses the j or J suffix to indicate the imaginary part (e.g. 3+5j).
The print() function produces a more readable output, by omitting the enclosing quotes and by printing escaped and special characters:
If you don’t want characters prefaced by \ to be interpreted as special characters, you can use raw strings by adding an r before the first quote:
>>> print(‘C:\some\name‘) # here \n means newline!C:\someame >>> print(r‘C:\some\name‘) # note the r before the quoteC:\some\name
Strings can be concatenated (glued together) with the + operator, and repeated with *:include literals,variables,expressions.
>>> # 3 times ‘un‘, followed by ‘ium‘ >>> 3 * ‘un‘ + ‘ium‘ ‘unununium‘
This feature is particularly useful when you want to break long strings:
>>> text = (‘Put several strings within parentheses ‘ ... ‘to have them joined together.‘) >>> text‘Put several strings within parentheses to have them joined together.‘
Strings can be indexed (subscripted), with the first character having index 0;Note that since -0 is the same as 0, negative indices start from -1.
>>> word = ‘Python‘ >>> word[0] # character in position 0 ‘P‘
>>> word[-1] # last character ‘n‘
In addition to indexing, slicing is also supported.
>>> word[0:2] # characters from position 0 (included) to 2 (excluded) ‘Py‘
Slice indices have useful defaults; an omitted first index defaults to zero, an omitted second index defaults to the size of the string being sliced.
>>> word[:2] # character from the beginning to position 2 (excluded) ‘Py‘ >>> word[4:] # characters from position 4 (included) to the end ‘on‘
Attempting to use an index that is too large will result in an error:
However, out of range slice indexes are handled gracefully when used for slicing:
>>> word[42] # the word only has 6 characters Traceback (most recent call last): File "<stdin>", line 1, in <module> IndexError: string index out of range
>>> word[4:42] ‘on‘ >>> word[42:] ‘‘
Python strings cannot be changed — they are immutable.If you need a different string, you should create a new one:
>>> word[0] = ‘J‘ ... TypeError: ‘str‘ object does not support item assignment
>>> ‘J‘ + word[1:] ‘Jython‘
The built-in function len() returns the length of a string:
Lists might contain items of different types, but usually the items all have the same type.
>>> squares = [1, 4, 9, 16, 25] >>> squares [1, 4, 9, 16, 25]
All slice operations return a new list containing the requested elements. This means that the following slice returns a new (shallow) copy of the list.
Unlike strings, which are immutable, lists are a mutable type
>>> cubes = [1, 8, 27, 65, 125] # something‘s wrong here >>> 4 ** 3 # the cube of 4 is 64, not 65!64 >>> cubes[3] = 64 # replace the wrong value >>> cubes [1, 8, 27, 64, 125]
You can also add new items at the end of the list, by using the append() method
>>> cubes.append(216) # add the cube of 6 >>> cubes.append(7 ** 3) # and the cube of 7 >>> cubes [1, 8, 27, 64, 125, 216, 343]
The built-in function len() also applies to lists:
It is possible to nest lists (create lists containing other lists).
The keyword argument end can be used to avoid the newline after the output, or end the output with a different string:
>>> a, b = 0, 1 >>> while b < 1000: ... print(b, end=‘,‘) ... a, b = b, a+b ... 1,1,2,3,5,8,13,21,34,55,89,144,233,377,610,987,
if StatementsThere can be zero or more elif parts, and the else part is optional. The keyword ‘elif‘ is short for ‘else if’, and is useful to avoid excessive indentation
>>> x = int(input("Please enter an integer: "))
Please enter an integer: 42
>>> if x < 0:
... x = 0
... print(‘Negative changed to zero‘)
... elif x == 0:
... print(‘Zero‘)
... elif x == 1:
... print(‘Single‘)
... else:
... print(‘More‘)
...
Morefor StatementsPython’s for statement iterates over the items of any sequence (a list or a string), in the order that they appear in the sequence.
>>> # Measure some strings: ... words = [‘cat‘, ‘window‘, ‘defenestrate‘] >>> for w in words: ... print(w, len(w)) ... cat 3 window 6 defenestrate 12
If you need to modify the sequence you are iterating over while inside the loop, it is recommended that you first make a copy. Iterating over a sequence does not implicitly make a copy. The slice notation makes this especially convenient:
>>> for w in words[:]: # Loop over a slice copy of the entire list .... if len(w) > 6: ... words.insert(0, w) ...> >> words [‘defenestrate‘, ‘cat‘, ‘window‘, ‘defenestrate‘]
With for w in words:, the example would attempt to create an infinite list, inserting defenestrate over and over again.
标签：python
原文地址：http://12261659.blog.51cto.com/12251659/1918412
