Python map()函数

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一、Python map()函数的用法

map(function, iterable)

 功能：遍历序列，对序列中每个元素进行操作，最终获取新的序列。

i = [11, 22, 33, 44, 55]
 
 map(函数，可迭代的对象(可以for循环的东西))
 def f2(a):
     return a + 100
 
 result = map(f2, li)
 result = map(lambda a: a + 200, li)
 print(list(result))
 
 filter  # 函数返回True，将元素添加到结果中
 map     # 将函数返回值添加到结果中
 
 def f1(args):
     result = []
     for i in args:
         result.append(100+i)
 
     return result
 r = f1(li)
 print(list(r))

输出结构如下：

[211, 222, 233, 244, 255]
[111, 122, 133, 144, 155]

应用场景：

1、每个元素增加100

2、两个列表对应元素相加

注意：map()函数不改变原有的 list，而是返回一个新的 list。

def format_name(s):
 s1=s[0:1].upper()+s[1:].lower();
 return s1;
print map(format_name, ['adam', 'LISA', 'barT'])

二、通过PyCharm查看源码

选中map函数右键Go To—Implementation代码如下

class map(object):
    """
    map(func, *iterables) --> map object
    
    Make an iterator that computes the function using arguments from
    each of the iterables.  Stops when the shortest iterable is exhausted.
    """
    def __getattribute__(self, *args, **kwargs): # real signature unknown
        """ Return getattr(self, name). """
        pass

    def __init__(self, func, *iterables): # real signature unknown; restored from __doc__
        pass

    def __iter__(self, *args, **kwargs): # real signature unknown
        """ Implement iter(self). """
        pass

    @staticmethod # known case of __new__
    def __new__(*args, **kwargs): # real signature unknown
        """ Create and return a new object.  See help(type) for accurate signature. """
        pass

    def __next__(self, *args, **kwargs): # real signature unknown
        """ Implement next(self). """
        pass

    def __reduce__(self, *args, **kwargs): # real signature unknown
        """ Return state information for pickling. """
        pass

三、查看源码里的FixMap.py

# Copyright 2007 Google, Inc. All Rights Reserved.
# Licensed to PSF under a Contributor Agreement.

"""Fixer that changes map(F, ...) into list(map(F, ...)) unless there
exists a 'from future_builtins import map' statement in the top-level
namespace.

As a special case, map(None, X) is changed into list(X).  (This is
necessary because the semantics are changed in this case -- the new
map(None, X) is equivalent to [(x,) for x in X].)

We avoid the transformation (except for the special case mentioned
above) if the map() call is directly contained in iter(<>), list(<>),
tuple(<>), sorted(<>), ...join(<>), or for V in <>:.

NOTE: This is still not correct if the original code was depending on
map(F, X, Y, ...) to go on until the longest argument is exhausted,
substituting None for missing values -- like zip(), it now stops as
soon as the shortest argument is exhausted.
"""

# Local imports
from ..pgen2 import token
from .. import fixer_base
from ..fixer_util import Name, ArgList, Call, ListComp, in_special_context
from ..pygram import python_symbols as syms
from ..pytree import Node


class FixMap(fixer_base.ConditionalFix):
    BM_compatible = True

    PATTERN = """
    map_none=power<
        'map'
        trailer< '(' arglist< 'None' ',' arg=any [','] > ')' >
        [extra_trailers=trailer*]
    >
    |
    map_lambda=power<
        'map'
        trailer<
            '('
            arglist<
                lambdef< 'lambda'
                         (fp=NAME | vfpdef< '(' fp=NAME ')'> ) ':' xp=any
                >
                ','
                it=any
            >
            ')'
        >
        [extra_trailers=trailer*]
    >
    |
    power<
        'map' args=trailer< '(' [any] ')' >
        [extra_trailers=trailer*]
    >
    """

    skip_on = 'future_builtins.map'

    def transform(self, node, results):
        if self.should_skip(node):
            return

        trailers = []
        if 'extra_trailers' in results:
            for t in results['extra_trailers']:
                trailers.append(t.clone())

        if node.parent.type == syms.simple_stmt:
            self.warning(node, "You should use a for loop here")
            new = node.clone()
            new.prefix = ""
            new = Call(Name("list"), [new])
        elif "map_lambda" in results:
            new = ListComp(results["xp"].clone(),
                           results["fp"].clone(),
                           results["it"].clone())
            new = Node(syms.power, [new] + trailers, prefix="")

        else:
            if "map_none" in results:
                new = results["arg"].clone()
                new.prefix = ""
            else:
                if "args" in results:
                    args = results["args"]
                    if args.type == syms.trailer and \
                       args.children[1].type == syms.arglist and \
                       args.children[1].children[0].type == token.NAME and \
                       args.children[1].children[0].value == "None":
                        self.warning(node, "cannot convert map(None, ...) "
                                     "with multiple arguments because map() "
                                     "now truncates to the shortest sequence")
                        return

                    new = Node(syms.power, [Name("map"), args.clone()])
                    new.prefix = ""

                if in_special_context(node):
                    return None

            new = Node(syms.power, [Name("list"), ArgList([new])] + trailers)
            new.prefix = ""

        new.prefix = node.prefix
        return new

重点在这

什么是fix呀？

# Copyright 2006 Google, Inc. All Rights Reserved.
# Licensed to PSF under a Contributor Agreement.

"""Base class for fixers (optional, but recommended)."""

# Python imports
import itertools

# Local imports
from .patcomp import PatternCompiler
from . import pygram
from .fixer_util import does_tree_import

class BaseFix(object):

    """Optional base class for fixers.

    The subclass name must be FixFooBar where FooBar is the result of
    removing underscores and capitalizing the words of the fix name.
    For example, the class name for a fixer named 'has_key' should be
    FixHasKey.
    """

    PATTERN = None  # Most subclasses should override with a string literal
    pattern = None  # Compiled pattern, set by compile_pattern()
    pattern_tree = None # Tree representation of the pattern
    options = None  # Options object passed to initializer
    filename = None # The filename (set by set_filename)
    numbers = itertools.count(1) # For new_name()
    used_names = set() # A set of all used NAMEs
    order = "post" # Does the fixer prefer pre- or post-order traversal
    explicit = False # Is this ignored by refactor.py -f all?
    run_order = 5   # Fixers will be sorted by run order before execution
                    # Lower numbers will be run first.
    _accept_type = None # [Advanced and not public] This tells RefactoringTool
                        # which node type to accept when there's not a pattern.

    keep_line_order = False # For the bottom matcher: match with the
                            # original line order
    BM_compatible = False # Compatibility with the bottom matching
                          # module; every fixer should set this
                          # manually

    # Shortcut for access to Python grammar symbols
    syms = pygram.python_symbols

    def __init__(self, options, log):
        """Initializer.  Subclass may override.

        Args:
            options: a dict containing the options passed to RefactoringTool
            that could be used to customize the fixer through the command line.
            log: a list to append warnings and other messages to.
        """
        self.options = options
        self.log = log
        self.compile_pattern()

    def compile_pattern(self):
        """Compiles self.PATTERN into self.pattern.

        Subclass may override if it doesn't want to use
        self.{pattern,PATTERN} in .match().
        """
        if self.PATTERN is not None:
            PC = PatternCompiler()
            self.pattern, self.pattern_tree = PC.compile_pattern(self.PATTERN,
                                                                 with_tree=True)

    def set_filename(self, filename):
        """Set the filename.

        The main refactoring tool should call this.
        """
        self.filename = filename

    def match(self, node):
        """Returns match for a given parse tree node.

        Should return a true or false object (not necessarily a bool).
        It may return a non-empty dict of matching sub-nodes as
        returned by a matching pattern.

        Subclass may override.
        """
        results = {"node": node}
        return self.pattern.match(node, results) and results

    def transform(self, node, results):
        """Returns the transformation for a given parse tree node.

        Args:
          node: the root of the parse tree that matched the fixer.
          results: a dict mapping symbolic names to part of the match.

        Returns:
          None, or a node that is a modified copy of the
          argument node.  The node argument may also be modified in-place to
          effect the same change.

        Subclass *must* override.
        """
        raise NotImplementedError()

    def new_name(self, template="xxx_todo_changeme"):
        """Return a string suitable for use as an identifier

        The new name is guaranteed not to conflict with other identifiers.
        """
        name = template
        while name in self.used_names:
            name = template + str(next(self.numbers))
        self.used_names.add(name)
        return name

    def log_message(self, message):
        if self.first_log:
            self.first_log = False
            self.log.append("### In file %s ###" % self.filename)
        self.log.append(message)

    def cannot_convert(self, node, reason=None):
        """Warn the user that a given chunk of code is not valid Python 3,
        but that it cannot be converted automatically.

        First argument is the top-level node for the code in question.
        Optional second argument is why it can't be converted.
        """
        lineno = node.get_lineno()
        for_output = node.clone()
        for_output.prefix = ""
        msg = "Line %d: could not convert: %s"
        self.log_message(msg % (lineno, for_output))
        if reason:
            self.log_message(reason)

    def warning(self, node, reason):
        """Used for warning the user about possible uncertainty in the
        translation.

        First argument is the top-level node for the code in question.
        Optional second argument is why it can't be converted.
        """
        lineno = node.get_lineno()
        self.log_message("Line %d: %s" % (lineno, reason))

    def start_tree(self, tree, filename):
        """Some fixers need to maintain tree-wide state.
        This method is called once, at the start of tree fix-up.

        tree - the root node of the tree to be processed.
        filename - the name of the file the tree came from.
        """
        self.used_names = tree.used_names
        self.set_filename(filename)
        self.numbers = itertools.count(1)
        self.first_log = True

    def finish_tree(self, tree, filename):
        """Some fixers need to maintain tree-wide state.
        This method is called once, at the conclusion of tree fix-up.

        tree - the root node of the tree to be processed.
        filename - the name of the file the tree came from.
        """
        pass


class ConditionalFix(BaseFix):
    """ Base class for fixers which not execute if an import is found. """

    # This is the name of the import which, if found, will cause the test to be skipped
    skip_on = None

    def start_tree(self, *args):
        super(ConditionalFix, self).start_tree(*args)
        self._should_skip = None

    def should_skip(self, node):
        if self._should_skip is not None:
            return self._should_skip
        pkg = self.skip_on.split(".")
        name = pkg[-1]
        pkg = ".".join(pkg[:-1])
        self._should_skip = does_tree_import(pkg, name, node)
        return self._should_skip

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