最近在学习python，基础知识学习了一遍。我的学习习惯是：了解了基础语法后，去阅读开源代码（掌握优秀程序员的一些写代码习惯和手法），最后结合项目实践，实践的过程对某些细节进行深化。
想起android编译系统中有一些python脚本，下面对build_image.py进行简单分析。
编译系统中调用build_image.py
生成系统镜像的target在build\core\Makefile中，
#----------build\core\Makefile-------------# $(1): output filedefine build-systemimage-target @echo "Target system fs image: $(1)"
@mkdir -p $(dir $(1)) $(systemimage_intermediates) && rm -rf $(systemimage_intermediates)/system_image_info.txt $(call generate-userimage-prop-dictionary, $(systemimage_intermediates)/system_image_info.txt, skip_fsck=true) $(hide) PATH=$(foreach p,$(INTERNAL_USERIMAGES_BINARY_PATHS),$(p):)$$PATH \
./build/tools/releasetools/build_image.py \ $(TARGET_OUT) $(systemimage_intermediates)/system_image_info.txt $(1)
endef
其中，可以看出来主要是借助了build_image.py去生成镜像，/build/tools/releasetools/build_image.py $(TARGET_OUT) $(systemimage_intermediates)/system_image_info.txt $(1) ， 有三个参数，一个是作为输入的文件目录，system_image_info.txt，和最终生成的镜像名，例如system.img。
build_image.py分析
由于上面makefile中是在shell中直接调用build_image.py(不是import 模块)，所以__name__为__main__，会直接调用main函数，
#参数格式必须为3个# $(TARGET_OUT) system_image_info.txt output_filedef main(argv):
if len(argv) != 3: print __doc__
sys.exit(1)
in_dir = argv[0]
glob_dict_file = argv[1]
out_file = argv[2] #将system_image_info.txt中的内容保存到字典中，例如{'fs_type':'ext4',.....}
glob_dict = LoadGlobalDict(glob_dict_file) #basename获取文件名(不包括路径)，这里为system.img
image_filename = os.path.basename(out_file)
mount_point = ""
if image_filename == "system.img":
mount_point = "system"
elif image_filename == "userdata.img":
mount_point = "data"
elif image_filename == "cache.img":
mount_point = "cache"
elif image_filename == "vendor.img":
mount_point = "vendor"
else: print >> sys.stderr, "error: unknown image file name ", image_filename
exit(1) #print >>
#the first expression after the >> must evaluate to a “file-like” object,
#输入为system_image_info.txt内容字典和system
image_properties = ImagePropFromGlobalDict(glob_dict, mount_point) if not BuildImage(in_dir, image_properties, out_file): print >> sys.stderr, "error: failed to build %s from %s" % (out_file, in_dir)
exit(1)#当被直接python build_image.py执行时 __name__为__main__#当被其他import时，__name__是模块的名字，build_image#argv[0]是脚本名if __name__ == '__main__':
main(sys.argv[1:])
其中，system_image_info.txt一个例子如下所示，一些参数及其值，
fs_type=ext4system_size=1288491008userdata_size=5835325440cache_fs_type=ext4cache_size=268435456extfs_sparse_flag=-sselinux_fc=out/target/product/msm8916_64/root/file_contextsverity=trueverity_key=build/target/product/security/verityverity_signer_cmd=out/host/linux-x86/bin/verity_signersystem_verity_block_device=/dev/block/bootdevice/by-name/systemskip_fsck=true
LoadGlobalDict函数如下，将system_image_info.txt中的内容保存到字典中，例如{‘fs_type’:’ext4’,…..}
def LoadGlobalDict(filename):
"""Load "name=value" pairs from filename"""
d = {}
f = open(filename) #对文件对象的迭代
#文件中#不处理，是注释
#strip去除字符串两头，不包括内部的空格
#split将字符串分割成序列 str.split(sep=None, maxsplit=-1)
#If maxsplit is given, at most maxsplit splits are done (thus, the list will have at most maxsplit+1 elements).
#>>>'1,2,3'.split(',', maxsplit=1)
#['1', '2,3']
#所以用=分割，只有=前面和后面2个元素
for line in f:
line = line.strip() if not line or line.startswith("#"): continue
k, v = line.split("=", 1)
d[k] = v
f.close() return d
ImagePropFromGlobalDict函数，从全局字典(system_image_info.txt中有些东西不是都有用)取出需要的东西，返回一个新字典。
def ImagePropFromGlobalDict(glob_dict, mount_point):
"""Build an image property dictionary from the global dictionary.
Args:
glob_dict: the global dictionary from the build system.
mount_point: such as "system", "data" etc.
"""
d = {} def copy_prop(src_p, dest_p):
if src_p in glob_dict:
d[dest_p] = str(glob_dict[src_p]) #extfs_sparse_flag=-s
#skip_fsck=true
#selinux_fc=out/target/product/msm8916_64/root/file_contexts
#将common_props属性中的值保存到新建的字典d中，
common_props = ( "extfs_sparse_flag", "mkyaffs2_extra_flags", "selinux_fc", "skip_fsck",
) for p in common_props:
copy_prop(p, p) #添加'mount_point':'system'
d["mount_point"] = mount_point if mount_point == "system": #'fs_type':'ext4'
#'partition_size':'1288491008'
copy_prop("fs_type", "fs_type")
copy_prop("system_size", "partition_size") elif mount_point == "data":
copy_prop("fs_type", "fs_type")
copy_prop("userdata_size", "partition_size") elif mount_point == "cache":
copy_prop("cache_fs_type", "fs_type")
copy_prop("cache_size", "partition_size") elif mount_point == "vendor":
copy_prop("vendor_fs_type", "fs_type")
copy_prop("vendor_size", "partition_size") return d
最后调用BuildImage函数，其实就是组了一个命令，然后执行该命令(mkuserimg.sh -s in_dir out_file ext4 system 1288491008 out/target/product/msm8916_64/root/file_contexts)。
def BuildImage(in_dir, prop_dict, out_file):
"""Build an image to out_file from in_dir with property prop_dict.
Args:
in_dir: path of input directory.
prop_dict: property dictionary.
out_file: path of the output image file.
Returns:
True iff the image is built successfully.
"""
build_command = [] #字典的get方法，如果键不存在，没有任何异常，而得到None
fs_type = prop_dict.get("fs_type", "")
run_fsck = False
#文件系统以ext开头
#这里是ext4，走这里,其实就是组一个要执行的命令 mkuserimg.sh -s in_dir out_file ext4 system 1288491008 out/target/product/msm8916_64/root/file_contexts
if fs_type.startswith("ext"):
build_command = ["mkuserimg.sh"] if "extfs_sparse_flag" in prop_dict:
build_command.append(prop_dict["extfs_sparse_flag"])
run_fsck = True
build_command.extend([in_dir, out_file, fs_type,
prop_dict["mount_point"]]) if "partition_size" in prop_dict:
build_command.append(prop_dict["partition_size"]) if "selinux_fc" in prop_dict:
build_command.append(prop_dict["selinux_fc"]) else:
build_command = ["mkyaffs2image", "-f"] #split用于将字符串分割成list，没有参数表示用空格分割
if prop_dict.get("mkyaffs2_extra_flags", None):
build_command.extend(prop_dict["mkyaffs2_extra_flags"].split())
build_command.append(in_dir)
build_command.append(out_file) if "selinux_fc" in prop_dict:
build_command.append(prop_dict["selinux_fc"])
build_command.append(prop_dict["mount_point"]) #执行该命令mkuserimg.sh -s in_dir out_file ext4 system
exit_code = RunCommand(build_command) if exit_code != 0: return False# >>> os.path.dirname('c:\\Python\\a.txt') 返回目录# 'c:\\Python'# >>> os.path.basename('c:\\Python\\a.txt') 返回文件名# 'a.txt'# os.path.join连接目录与文件名或目录
#skip_fsck=true，不走这里
if run_fsck and prop_dict.get("skip_fsck") != "true": # Inflate the sparse image
unsparse_image = os.path.join(
os.path.dirname(out_file), "unsparse_" + os.path.basename(out_file))
inflate_command = ["simg2img", out_file, unsparse_image]
exit_code = RunCommand(inflate_command) #This function is semantically identical to unlink().
#remove和unlink一样
if exit_code != 0:
os.remove(unsparse_image) return False
# Run e2fsck on the inflated image file
e2fsck_command = ["e2fsck", "-f", "-n", unsparse_image]
exit_code = RunCommand(e2fsck_command)
os.remove(unsparse_image) return exit_code == 0
其中，RunCommand其实就类似于c语言中的system()，执行一段命令，获取返回值，用popen实现的。
def RunCommand(cmd):
""" Echo and run the given command
Args:
cmd: the command represented as a list of strings.
Returns:
The exit code.
"""
print "Running: ", " ".join(cmd) #创建子进程
p = subprocess.Popen(cmd) #等待子进程执行完
p.communicate() return p.returncode
最终其实是调用了mkuserimg.sh脚本，脚本核心是调用make_ext4fs 应用程序
#system\extras\ext4_utils\mkuserimg.shfunction usage() {
cat<<EOT
Usage:
mkuserimg.sh [-s] SRC_DIR OUTPUT_FILE EXT_VARIANT MOUNT_POINT SIZE [FILE_CONTEXTS]
EOT
}echo "in mkuserimg.sh PATH=$PATH"ENABLE_SPARSE_IMAGE=if [ "$1" = "-s" ]; then
ENABLE_SPARSE_IMAGE="-s"
shiftfiif [ $# -ne 5 -a $# -ne 6 ]; then
usage exit 1fiSRC_DIR=$1if [ ! -d $SRC_DIR ]; then
echo "Can not find directory $SRC_DIR!"
exit 2fiOUTPUT_FILE=$2EXT_VARIANT=$3MOUNT_POINT=$4SIZE=$5FC=$6case $EXT_VARIANT in
ext4) ;;
*) echo "Only ext4 is supported!"; exit 3 ;;esacif [ -z $MOUNT_POINT ]; then
echo "Mount point is required"
exit 2fiif [ -z $SIZE ]; then
echo "Need size of filesystem"
exit 2fiif [ -n "$FC" ]; then
FCOPT="-S $FC"fi#核心是make_ext4fs 应用程序MAKE_EXT4FS_CMD="make_ext4fs $ENABLE_SPARSE_IMAGE $FCOPT -l $SIZE -a $MOUNT_POINT $OUTPUT_FILE $SRC_DIR"echo $MAKE_EXT4FS_CMD$MAKE_EXT4FS_CMDif [ $? -ne 0 ]; then
exit 4fi
而make_ext4fs 应用程序具体在system\extras\ext4_utils中生成，makefile如下，其中makefile中有两个make_ext4fs的module，一个是host(BUILD_HOST_EXECUTABLE)，一个是target的(BUILD_EXECUTABLE)，这个命令是在编译机调用，也就是host上而不是target上。
include $(CLEAR_VARS)LOCAL_SRC_FILES := make_ext4fs_main.cLOCAL_MODULE := make_ext4fsLOCAL_STATIC_LIBRARIES += \
libext4_utils_host \
libsparse_host \
libz
ifeq ($(HOST_OS),windows) LOCAL_LDLIBS += -lws2_32else
LOCAL_STATIC_LIBRARIES += libselinux LOCAL_CFLAGS := -DHOSTendifinclude $(BUILD_HOST_EXECUTABLE)
