Merge pull request #17436 from vpisarev:fix_python_io

* fixed #17044
1. fixed Python part of the tutorial about using OpenCV XML-YAML-JSON I/O functionality from C++ and Python.
2. added startWriteStruct() and endWriteStruct() methods to FileStorage
3. modifed FileStorage::write() methods to make them work well inside sequences, not only mappings.

* try to fix the doc builder

* added Python regression test for FileStorage I/O API ([TODO] iterating through long sequences can be very slow)

* fixed yaml testing

modules/core/include/opencv2/core/persistence.hpp

@@ -429,7 +429,7 @@ public:
     /** @brief Writes the registered C structure (CvMat, CvMatND, CvSeq).
     @param name Name of the written object.
     @param obj Pointer to the object.
-    @see ocvWrite for details.
+    @see cvWrite for details.
      */
     void writeObj( const String& name, const void* obj );
 
@@ -456,6 +456,17 @@ public:
      */
     CV_WRAP void writeComment(const String& comment, bool append = false);
 
+    /** @brief Starts to write a nested structure (sequence or a mapping).
+    @param name name of the structure (if it's a member of parent mapping, otherwise it should be empty
+    @param flags type of the structure (FileNode::MAP or FileNode::SEQ (both with optional FileNode::FLOW)).
+    @param typeName usually an empty string
+    */
+    CV_WRAP void startWriteStruct(const String& name, int flags, const String& typeName=String());
+
+    /** @brief Finishes writing nested structure (should pair startWriteStruct())
+    */
+    CV_WRAP void endWriteStruct();
+
     /** @brief Returns the normalized object name for the specified name of a file.
     @param filename Name of a file
     @returns The normalized object name.

modules/core/src/persistence_cpp.cpp

@@ -166,22 +166,24 @@ void FileStorage::writeObj( const String& name, const void* obj )
 
 void FileStorage::write( const String& name, int val )
 {
-    *this << name << val;
+    cvWriteInt(fs, name.c_str(), val);
 }
 
 void FileStorage::write( const String& name, double val )
 {
-    *this << name << val;
+    cvWriteReal(fs, name.c_str(), val);
 }
 
 void FileStorage::write( const String& name, const String& val )
 {
-    *this << name << val;
+    cvWriteString(fs, name.c_str(), val.c_str());
 }
 
 void FileStorage::write( const String& name, InputArray val )
 {
-    *this << name << val.getMat();
+    if(state & INSIDE_MAP)
+        *this << name;
+    *this << val.getMat();
 }
 
 void FileStorage::writeComment( const String& comment, bool append )
@@ -189,6 +191,26 @@ void FileStorage::writeComment( const String& comment, bool append )
     cvWriteComment(fs, comment.c_str(), append ? 1 : 0);
 }
 
+void FileStorage::startWriteStruct(const String& name, int flags, const String& typeName)
+{
+    int struct_type = flags & FileNode::TYPE_MASK;
+    bool isflow = (flags & FileNode::FLOW) != 0;
+    CV_Assert(struct_type == FileNode::SEQ || struct_type == FileNode::MAP);
+    char strbegin_[] = { (struct_type == FileNode::SEQ ? '[' : '{'), (isflow ? ':' : '\0'), '\0' };
+    String strbegin = strbegin_;
+    if (!typeName.empty())
+        strbegin += typeName;
+    *this << name << strbegin;
+}
+
+void FileStorage::endWriteStruct()
+{
+    if( structs.empty() )
+        CV_Error( CV_StsError, "Extra endWriteStruct()" );
+    char openparen = structs.back();
+    *this << (openparen == '[' ? "]" : "}");
+}
+
 String FileStorage::getDefaultObjectName(const String& _filename)
 {
     static const char* stubname = "unnamed";

modules/core/src/persistence_xml.cpp

@@ -853,7 +853,7 @@ void icvXMLWriteScalar( CvFileStorage* fs, const char* key, const char* data, in
         char* ptr = fs->buffer;
         int new_offset = (int)(ptr - fs->buffer_start) + len;
 
-        if( key )
+        if( key && key[0] != '\0' )
             CV_Error( CV_StsBadArg, "elements with keys can not be written to sequence" );
 
         fs->struct_flags = CV_NODE_SEQ;