为什么我在一个案例中得到一个KeyError，而在另一个案例中却没有？见下文。
>>> import pandas as pd
>>> s1 = pd.Series([10, 20, 30, 40], index=list('abcd'))
>>> s2 = pd.Series([10, 20, 30, 40], index=list('acbd'))
>>> s1.loc['a':'z']
a 10
b 20
c 30
d 40
dtype: int64
>>> s2.loc['a':'z']
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/path/lib/python2.7/site-packages/pandas/core/series.py", line 559, in __getitem__
return self._get_with(key)
File "/path/lib/python2.7/site-packages/pandas/core/series.py", line 564, in _get_with
indexer = self.index._convert_slice_indexer(key, kind='getitem')
File "/path/lib/python2.7/site-packages/pandas/core/index.py", line 871, in _convert_slice_indexer
key, is_index_slice=is_index_slice)
File "/path/lib/python2.7/site-packages/pandas/core/index.py", line 806, in _convert_slice_indexer_getitem
return self._convert_slice_indexer(key)
File "/path/lib/python2.7/site-packages/pandas/core/index.py", line 893, in _convert_slice_indexer
indexer = self.slice_indexer(start, stop, step)
File "/path/lib/python2.7/site-packages/pandas/core/index.py", line 2346, in slice_indexer
start_slice, end_slice = self.slice_locs(start, end, step=step, kind=kind)
File "/path/lib/python2.7/site-packages/pandas/core/index.py", line 2496, in slice_locs
end_slice = self.get_slice_bound(end, 'right', kind)
File "/path/lib/python2.7/site-packages/pandas/core/index.py", line 2438, in get_slice_bound
raise err
KeyError: 'z'
更新：
其中一个答案说这可能是因为s1.index.is_monotonic是真的。但是，is_monotonic为False的情况属于KeyError，但没有>>> s3 = pd.Series([10, 20, 30, 40], index=list('dcba'))
>>> s3['z':'a']
d 10
c 20
b 30
a 40
dtype: int64
>>> s3.index.is_monotonic
False
。
class Preview extends SurfaceView implements SurfaceHolder.Callback, PreviewCallback {
public interface PreviewListener {
public void OnPreviewUpdated(int[] pixels, int width, int height);
}
PreviewListener listener;
SurfaceHolder mHolder;
Camera mCamera = null;
byte[] buffer;
int bufferSize;
private boolean isFrontCamera = false;
boolean lightOn = false;
private boolean isPaused = false;
//This variable is responsible for getting and setting the camera settings
private Parameters parameters;
//this variable stores the camera preview size
private Size previewSize;
//this array stores the pixels as hexadecimal pairs
private int[] pixels;
Preview(Context context) {
super(context);
// Install a SurfaceHolder.Callback so we get notified when the
// underlying surface is created and destroyed.
mHolder = getHolder();
mHolder.addCallback(this);
mHolder.setType(SurfaceHolder.SURFACE_TYPE_PUSH_BUFFERS);
try {
// Instantiate the EnterColorListener so we can send events to the host
listener = (PreviewListener) context;
} catch (ClassCastException e) {
// The activity doesn't implement the interface, throw exception
throw new ClassCastException(context.toString()
+ " must implement PreviewListener");
}
}
public void surfaceCreated(SurfaceHolder holder) {
// The Surface has been created, acquire the camera and tell it where
// to draw.
try {
CameraInfo info = new CameraInfo();
Camera.getCameraInfo(0, info);
if (info.facing == CameraInfo.CAMERA_FACING_FRONT) {
this.isFrontCamera = true;
}
mCamera = Camera.open(0); // attempt to get a Camera instance. index b/c front cameras are ok too.
}
catch (Exception e){
// Camera is not available (in use or does not exist)
Log.e("camera error", "could not open camera");
return;
}
try {
mCamera.setDisplayOrientation(90);
mCamera.setPreviewDisplay(holder);
} catch (IOException exception) {
mCamera.release();
mCamera = null;
}
}
// thanks cheatcoder@github
public void flash() {
if (supportsFlash()) {
if (!lightOn) {
lightOn = true;
mCamera.stopPreview();
mCamera.setPreviewCallbackWithBuffer(this);// mCamera.setPreviewCallback(this);//setPreviewCallbackWithBuffer(this);
parameters.setFlashMode(Parameters.FLASH_MODE_TORCH);
mCamera.setParameters(parameters);
mCamera.startPreview();
} else {
lightOn = false;
mCamera.stopPreview();
mCamera.setPreviewCallbackWithBuffer(this); //setPreviewCallback(this);//setPreviewCallbackWithBuffer(this);
parameters.setFlashMode(Parameters.FLASH_MODE_OFF);
mCamera.setParameters(parameters);
mCamera.startPreview();
}
}
}
// thanks http://ikravchenko.blogspot.com/2013/09/nexus-7-2013-torch-issue.html
public boolean supportsFlash() {
if (getContext().getPackageManager().hasSystemFeature(PackageManager.FEATURE_CAMERA_FLASH)) {
parameters = mCamera.getParameters();
if (parameters.getFlashMode() != null) {
List<String> supportedFlashModes = parameters.getSupportedFlashModes();
if (supportedFlashModes == null || supportedFlashModes.isEmpty() || supportedFlashModes.size() == 1 && supportedFlashModes.get(0).equals(Camera.Parameters.FLASH_MODE_OFF)) {
return false;
}
return true;
}
}
return false;
}
public void surfaceDestroyed(SurfaceHolder holder) {
// Surface will be destroyed when we return, so stop the preview.
// Because the CameraDevice object is not a shared resource, it's very
// important to release it when the activity is paused.
if (mCamera != null) {
mCamera.stopPreview();
mCamera.setPreviewCallback(null);//setPreviewCallbackWithBuffer(null);
mCamera.release();
mCamera = null;
}
}
public boolean isFrontCamera() {
return isFrontCamera;
}
/* TODO: fix the bug with the null pointer exception */
public void surfaceChanged(SurfaceHolder holder, int format, int w, int h) {
// Now that the size is known, set up the camera parameters and begin
// the preview.
if (mCamera != null) {
parameters = mCamera.getParameters();
//to do autofocus, need to set the parameters if available
//http://stackoverflow.com/questions/11623266/camera-parameters-setfocusmode-is-not-working
List<String> focusModes = parameters.getSupportedFocusModes();
if (focusModes != null) {
if (focusModes.contains(Parameters.FOCUS_MODE_CONTINUOUS_PICTURE))
parameters.setFocusMode(Parameters.FOCUS_MODE_CONTINUOUS_PICTURE);
else if (focusModes.contains(Parameters.FOCUS_MODE_CONTINUOUS_VIDEO))
parameters.setFocusMode(Parameters.FOCUS_MODE_CONTINUOUS_VIDEO);
else if (focusModes.contains(Parameters.FOCUS_MODE_AUTO))
parameters.setFocusMode(Parameters.FOCUS_MODE_AUTO);
}
//have to get previewSizes because not all devices support arbitrary previews
//the following is from Stack Overflow
int width = this.getWidth();
int height = this.getHeight();
Size best = null;
List<Camera.Size> previewSizes = parameters.getSupportedPreviewSizes();
// You need to choose the most appropriate previewSize for your app
for (int i = 0; i < previewSizes.size(); i++) {
Size size = previewSizes.get(i);
if ((size.width <= width && size.height <= height) || (size.height <= width && size.width <= height)) {
if (best==null) {
best=size;
} else {
int resultArea=best.width*best.height;
int newArea=size.width*size.height;
if (newArea>resultArea) {
best=size;
}
}
}
}
// make sure something is picked. previewSizes is guarenteed to have at least one thing.
if (best != null) {
previewSize = best;
} else {
previewSize = previewSizes.get(0);
}
parameters.setPreviewSize(previewSize.width, previewSize.height);
pixels = new int[previewSize.width * previewSize.height];
mCamera.setParameters(parameters);
//sets the camera callback to be the one defined in this class
mCamera.setPreviewCallbackWithBuffer(this);//setPreviewCallback(this);//setPreviewCallbackWithBuffer(this);
bufferSize = previewSize.width*previewSize.height*ImageFormat.getBitsPerPixel(parameters.getPreviewFormat())/8;
buffer = new byte[bufferSize];
resetBuffer();
if (!isPaused) mCamera.startPreview();
}
}
public void pause(boolean isPaused) {
this.isPaused = isPaused;
if (isPaused) {
parameters.setFlashMode(Parameters.FLASH_MODE_OFF);
mCamera.setParameters(parameters);
mCamera.stopPreview();
} else {
if (mCamera != null) {
mCamera.setPreviewCallbackWithBuffer(this);//setPreviewCallback(this);//setPreviewCallbackWithBuffer(this);
if(lightOn)
parameters.setFlashMode(Parameters.FLASH_MODE_TORCH);
mCamera.setParameters(parameters);
mCamera.startPreview();
}
}
}
public void resetBuffer() {
if (mCamera != null) {
mCamera.addCallbackBuffer(buffer);
}
}
@Override
public void onPreviewFrame(byte[] data, Camera camera) {
//transforms NV21 pixel data into RGB pixels
decodeYUV420SP(pixels, data, previewSize.width, previewSize.height);
listener.OnPreviewUpdated(pixels, previewSize.width, previewSize.height);
}
//Method from Ketai project! Not mine! See below...
void decodeYUV420SP(int[] rgb, byte[] yuv420sp, int width, int height) {
final int frameSize = width * height;
for (int j = 0, yp = 0; j < height; j++) { int uvp = frameSize + (j >> 1) * width, u = 0, v = 0;
for (int i = 0; i < width; i++, yp++) {
int y = (0xff & ((int) yuv420sp[yp])) - 16;
if (y < 0)
y = 0;
if ((i & 1) == 0) {
v = (0xff & yuv420sp[uvp++]) - 128;
u = (0xff & yuv420sp[uvp++]) - 128;
}
int y1192 = 1192 * y;
int r = (y1192 + 1634 * v);
int g = (y1192 - 833 * v - 400 * u);
int b = (y1192 + 2066 * u);
if (r < 0) r = 0; else if (r > 262143)
r = 262143;
if (g < 0) g = 0; else if (g > 262143)
g = 262143;
if (b < 0) b = 0; else if (b > 262143)
b = 262143;
rgb[yp] = 0xff000000 | ((r << 6) & 0xff0000) | ((g >> 2) & 0xff00) | ((b >> 10) & 0xff);
}
}
}
}
Python v2.7.5，pandas v0.16.1
答案 0 :(得分：1)
我认为这归结为t的索引不是单调的，即它没有排序。排序/单调索引允许一些对未排序的行为不可能的行为。
s.index.is_monotonic
Out[15]: True
t.index.is_monotonic
Out[16]: False
docs on multi-indexing中简要提到了这一点，虽然显然在您的情况下并不适用，但我不知道在哪里可以很好地了解切片如何适用于排序/未分类的指数。